diff --git a/polkadot/network/src/tests/validation.rs b/polkadot/network/src/tests/validation.rs
index 428d86e019..a8903edd51 100644
--- a/polkadot/network/src/tests/validation.rs
+++ b/polkadot/network/src/tests/validation.rs
@@ -413,5 +413,6 @@ fn make_table(data: &ApiData, local_key: &AuthorityKeyring, parent_hash: Hash) -
 		Arc::new(local_key.pair()),
 		parent_hash,
 		store,
+		None,
 	))
 }
diff --git a/polkadot/service/src/lib.rs b/polkadot/service/src/lib.rs
index d568b908bc..343b2a291a 100644
--- a/polkadot/service/src/lib.rs
+++ b/polkadot/service/src/lib.rs
@@ -85,6 +85,9 @@ pub struct CustomConfiguration {
 		grandpa::LinkHalfForService<Factory>
 	)>,
 
+	/// Maximal `block_data` size.
+	pub max_block_data_size: Option<u64>,
+
 	inherent_data_providers: InherentDataProviders,
 }
 
@@ -94,6 +97,7 @@ impl Default for CustomConfiguration {
 			collating_for: None,
 			grandpa_import_setup: None,
 			inherent_data_providers: InherentDataProviders::new(),
+			max_block_data_size: None,
 		}
 	}
 }
@@ -311,6 +315,7 @@ construct_service_factory! {
 					key.clone(),
 					extrinsic_store,
 					SlotDuration::get_or_compute(&*client)?,
+					service.config.custom.max_block_data_size,
 				);
 
 				info!("Using authority key {}", key.public());
diff --git a/polkadot/validation/src/attestation_service.rs b/polkadot/validation/src/attestation_service.rs
index 76a0fce1cb..4b9f53af08 100644
--- a/polkadot/validation/src/attestation_service.rs
+++ b/polkadot/validation/src/attestation_service.rs
@@ -110,6 +110,7 @@ pub(crate) fn start<C, N, P, SC>(
 	thread_pool: TaskExecutor,
 	key: Arc<ed25519::Pair>,
 	extrinsic_store: ExtrinsicStore,
+	max_block_data_size: Option<u64>,
 ) -> ServiceHandle
 	where
 		C: Collators + Send + Sync + 'static,
@@ -147,6 +148,7 @@ pub(crate) fn start<C, N, P, SC>(
 									notification.header.parent_hash().clone(),
 									&authorities,
 									key.clone(),
+									max_block_data_size,
 								)
 							});
 
diff --git a/polkadot/validation/src/collation.rs b/polkadot/validation/src/collation.rs
index 0ec3510671..d0c8632b84 100644
--- a/polkadot/validation/src/collation.rs
+++ b/polkadot/validation/src/collation.rs
@@ -64,6 +64,7 @@ pub struct CollationFetch<C: Collators, P> {
 	collators: C,
 	live_fetch: Option<<C::Collation as IntoFuture>::Future>,
 	client: Arc<P>,
+	max_block_data_size: Option<u64>,
 }
 
 impl<C: Collators, P> CollationFetch<C, P> {
@@ -73,6 +74,7 @@ impl<C: Collators, P> CollationFetch<C, P> {
 		relay_parent_hash: Hash,
 		collators: C,
 		client: Arc<P>,
+		max_block_data_size: Option<u64>,
 	) -> Self {
 		CollationFetch {
 			relay_parent: BlockId::hash(relay_parent_hash),
@@ -81,6 +83,7 @@ impl<C: Collators, P> CollationFetch<C, P> {
 			client,
 			parachain,
 			live_fetch: None,
+			max_block_data_size,
 		}
 	}
 
@@ -113,7 +116,7 @@ impl<C: Collators, P: ProvideRuntimeApi> Future for CollationFetch<C, P>
 				try_ready!(poll)
 			};
 
-			let res = validate_collation(&*self.client, &self.relay_parent, &collation);
+			let res = validate_collation(&*self.client, &self.relay_parent, &collation, self.max_block_data_size);
 
 			match res {
 				Ok(e) => {
@@ -179,7 +182,11 @@ error_chain! {
 		}
 		WrongHeadData(expected: Vec<u8>, got: Vec<u8>) {
 			description("Parachain validation produced wrong head data."),
-			display("Parachain validation produced wrong head data (expected: {:?}, got {:?}", expected, got),
+			display("Parachain validation produced wrong head data (expected: {:?}, got {:?})", expected, got),
+		}
+		BlockDataTooBig(size: u64, max_size: u64) {
+			description("Block data is too big."),
+			display("Block data is too big (maximum allowed size: {}, actual size: {})", max_size, size),
 		}
 	}
 }
@@ -331,12 +338,20 @@ pub fn validate_collation<P>(
 	client: &P,
 	relay_parent: &BlockId,
 	collation: &Collation,
+	max_block_data_size: Option<u64>,
 ) -> Result<Extrinsic, Error> where
 	P: ProvideRuntimeApi,
 	P::Api: ParachainHost<Block>,
 {
 	use parachain::{IncomingMessage, ValidationParams};
 
+	if let Some(max_size) = max_block_data_size {
+		let block_data_size = collation.pov.block_data.0.len() as u64;
+		if block_data_size > max_size {
+			return Err(ErrorKind::BlockDataTooBig(block_data_size, max_size).into());
+		}
+	}
+
 	let api = client.runtime_api();
 	let para_id = collation.receipt.parachain_index;
 	let validation_code = api.parachain_code(relay_parent, para_id)?
diff --git a/polkadot/validation/src/lib.rs b/polkadot/validation/src/lib.rs
index b7c5e93dcf..fbb98b9556 100644
--- a/polkadot/validation/src/lib.rs
+++ b/polkadot/validation/src/lib.rs
@@ -300,6 +300,7 @@ impl<C, N, P> ParachainValidation<C, N, P> where
 		grandparent_hash: Hash,
 		authorities: &[AuthorityId],
 		sign_with: Arc<ed25519::Pair>,
+		max_block_data_size: Option<u64>,
 	)
 		-> Result<Arc<AttestationTracker>, Error>
 	{
@@ -351,7 +352,14 @@ impl<C, N, P> ParachainValidation<C, N, P> where
 
 		debug!(target: "validation", "Active parachains: {:?}", active_parachains);
 
-		let table = Arc::new(SharedTable::new(authorities, group_info, sign_with.clone(), parent_hash, self.extrinsic_store.clone()));
+		let table = Arc::new(SharedTable::new(
+			authorities,
+			group_info,
+			sign_with.clone(),
+			parent_hash,
+			self.extrinsic_store.clone(),
+			max_block_data_size,
+		));
 		let router = self.network.communication_for(
 			table.clone(),
 			authorities,
@@ -362,6 +370,7 @@ impl<C, N, P> ParachainValidation<C, N, P> where
 				parent_hash,
 				id,
 				router,
+				max_block_data_size,
 			)),
 			Chain::Relay => None,
 		};
@@ -387,7 +396,8 @@ impl<C, N, P> ParachainValidation<C, N, P> where
 		&self,
 		relay_parent: Hash,
 		validation_para: ParaId,
-		build_router: N::BuildTableRouter
+		build_router: N::BuildTableRouter,
+		max_block_data_size: Option<u64>,
 	) -> exit_future::Signal {
 		use extrinsic_store::Data;
 
@@ -402,6 +412,7 @@ impl<C, N, P> ParachainValidation<C, N, P> where
 				relay_parent,
 				collators,
 				client,
+				max_block_data_size,
 			);
 
 			collation_work.then(move |result| match result {
@@ -466,6 +477,7 @@ pub struct ProposerFactory<C, N, P, SC, TxApi: PoolChainApi> {
 	_service_handle: ServiceHandle,
 	aura_slot_duration: SlotDuration,
 	select_chain: SC,
+	max_block_data_size: Option<u64>,
 }
 
 impl<C, N, P, SC, TxApi> ProposerFactory<C, N, P, SC, TxApi> where
@@ -491,6 +503,7 @@ impl<C, N, P, SC, TxApi> ProposerFactory<C, N, P, SC, TxApi> where
 		key: Arc<ed25519::Pair>,
 		extrinsic_store: ExtrinsicStore,
 		aura_slot_duration: SlotDuration,
+		max_block_data_size: Option<u64>,
 	) -> Self {
 		let parachain_validation = Arc::new(ParachainValidation {
 			client: client.clone(),
@@ -508,6 +521,7 @@ impl<C, N, P, SC, TxApi> ProposerFactory<C, N, P, SC, TxApi> where
 			thread_pool,
 			key.clone(),
 			extrinsic_store,
+			max_block_data_size,
 		);
 
 		ProposerFactory {
@@ -516,7 +530,8 @@ impl<C, N, P, SC, TxApi> ProposerFactory<C, N, P, SC, TxApi> where
 			key,
 			_service_handle: service_handle,
 			aura_slot_duration,
-			select_chain
+			select_chain,
+			max_block_data_size,
 		}
 	}
 }
@@ -548,6 +563,7 @@ impl<C, N, P, SC, TxApi> consensus::Environment<Block> for ProposerFactory<C, N,
 			parent_header.parent_hash().clone(),
 			authorities,
 			sign_with,
+			self.max_block_data_size,
 		)?;
 
 		Ok(Proposer {
diff --git a/polkadot/validation/src/shared_table/mod.rs b/polkadot/validation/src/shared_table/mod.rs
index ccddaba955..5c6293cbf1 100644
--- a/polkadot/validation/src/shared_table/mod.rs
+++ b/polkadot/validation/src/shared_table/mod.rs
@@ -136,6 +136,7 @@ impl SharedTableInner {
 		context: &TableContext,
 		router: &R,
 		statement: table::SignedStatement,
+		max_block_data_size: Option<u64>,
 	) -> Option<ParachainWork<
 		<R::FetchValidationProof as IntoFuture>::Future,
 	>> {
@@ -191,7 +192,8 @@ impl SharedTableInner {
 		work.map(|work| ParachainWork {
 			extrinsic_store: self.extrinsic_store.clone(),
 			relay_parent: context.parent_hash.clone(),
-			work
+			work,
+			max_block_data_size,
 		})
 	}
 
@@ -265,6 +267,7 @@ pub struct ParachainWork<Fetch> {
 	work: Work<Fetch>,
 	relay_parent: Hash,
 	extrinsic_store: ExtrinsicStore,
+	max_block_data_size: Option<u64>,
 }
 
 impl<Fetch: Future> ParachainWork<Fetch> {
@@ -279,11 +282,13 @@ impl<Fetch: Future> ParachainWork<Fetch> {
 			P: Send + Sync + 'static,
 			P::Api: ParachainHost<Block>,
 	{
+		let max_block_data_size = self.max_block_data_size;
 		let validate = move |id: &_, collation: &_| {
 			let res = ::collation::validate_collation(
 				&*api,
 				id,
 				collation,
+				max_block_data_size,
 			);
 
 			match res {
@@ -373,13 +378,15 @@ impl<Fetch, F, Err> Future for PrimedParachainWork<Fetch, F>
 pub struct SharedTable {
 	context: Arc<TableContext>,
 	inner: Arc<Mutex<SharedTableInner>>,
+	max_block_data_size: Option<u64>,
 }
 
 impl Clone for SharedTable {
 	fn clone(&self) -> Self {
-		SharedTable {
+		Self {
 			context: self.context.clone(),
 			inner: self.inner.clone(),
+			max_block_data_size: self.max_block_data_size,
 		}
 	}
 }
@@ -395,10 +402,12 @@ impl SharedTable {
 		key: Arc<ed25519::Pair>,
 		parent_hash: Hash,
 		extrinsic_store: ExtrinsicStore,
+		max_block_data_size: Option<u64>,
 	) -> Self {
 		let index_mapping = authorities.iter().enumerate().map(|(i, k)| (i as ValidatorIndex, k.clone())).collect();
 		Self {
 			context: Arc::new(TableContext { groups, key, parent_hash, index_mapping, }),
+			max_block_data_size,
 			inner: Arc::new(Mutex::new(SharedTableInner {
 				table: Table::default(),
 				validated: HashMap::new(),
@@ -447,7 +456,7 @@ impl SharedTable {
 	) -> Option<ParachainWork<
 		<R::FetchValidationProof as IntoFuture>::Future,
 	>> {
-		self.inner.lock().import_remote_statement(&*self.context, router, statement)
+		self.inner.lock().import_remote_statement(&*self.context, router, statement, self.max_block_data_size)
 	}
 
 	/// Import many statements at once.
@@ -467,7 +476,7 @@ impl SharedTable {
 		let mut inner = self.inner.lock();
 
 		iterable.into_iter().map(move |statement| {
-			inner.import_remote_statement(&*self.context, router, statement)
+			inner.import_remote_statement(&*self.context, router, statement, self.max_block_data_size)
 		}).collect()
 	}
 
@@ -612,6 +621,7 @@ mod tests {
 			local_key.clone(),
 			parent_hash,
 			ExtrinsicStore::new_in_memory(),
+			None,
 		);
 
 		let candidate = CandidateReceipt {
@@ -665,6 +675,7 @@ mod tests {
 			local_key.clone(),
 			parent_hash,
 			ExtrinsicStore::new_in_memory(),
+			None,
 		);
 
 		let candidate = CandidateReceipt {
@@ -720,6 +731,7 @@ mod tests {
 			},
 			relay_parent,
 			extrinsic_store: store.clone(),
+			max_block_data_size: None,
 		};
 
 		let validated = producer.prime_with(|_, _| Ok(Extrinsic { outgoing_messages: Vec::new() }))
@@ -760,6 +772,7 @@ mod tests {
 			},
 			relay_parent,
 			extrinsic_store: store.clone(),
+			max_block_data_size: None,
 		};
 
 		let validated = producer.prime_with(|_, _| Ok(Extrinsic { outgoing_messages: Vec::new() }))
@@ -797,6 +810,7 @@ mod tests {
 			local_key.clone(),
 			parent_hash,
 			ExtrinsicStore::new_in_memory(),
+			None,
 		);
 
 		let candidate = CandidateReceipt {
@@ -861,6 +875,7 @@ mod tests {
 			local_key.clone(),
 			parent_hash,
 			ExtrinsicStore::new_in_memory(),
+			None,
 		);
 
 		let candidate = CandidateReceipt {
@@ -909,6 +924,7 @@ mod tests {
 				Arc::new(AuthorityKeyring::Alice.pair()),
 				Default::default(),
 				ExtrinsicStore::new_in_memory(),
+				None,
 			);
 			let expected_mapping = authorities.iter().enumerate().map(|(i, k)| (i as ValidatorIndex, k.clone())).collect();
 			assert_eq!(shared_table.context.index_mapping, expected_mapping);