diff --git a/zebra-state/Cargo.toml b/zebra-state/Cargo.toml
index 610c0064..7fabe6e4 100644
--- a/zebra-state/Cargo.toml
+++ b/zebra-state/Cargo.toml
@@ -23,6 +23,7 @@ tower = "0.3.1"
 tracing = "0.1"
 tracing-error = "0.1.2"
 thiserror = "1.0.20"
+tokio = { version = "0.2.22", features = ["sync"] }
 
 [dev-dependencies]
 zebra-test = { path = "../zebra-test/" }
diff --git a/zebra-state/src/lib.rs b/zebra-state/src/lib.rs
index 559bbc87..ebcea597 100644
--- a/zebra-state/src/lib.rs
+++ b/zebra-state/src/lib.rs
@@ -20,6 +20,7 @@ mod util;
 mod tests;
 
 use memory_state::MemoryState;
+use service::QueuedBlock;
 use sled_state::SledState;
 
 pub use config::Config;
diff --git a/zebra-state/src/service.rs b/zebra-state/src/service.rs
index 51369764..36d1d2bf 100644
--- a/zebra-state/src/service.rs
+++ b/zebra-state/src/service.rs
@@ -1,15 +1,29 @@
 use std::{
     future::Future,
     pin::Pin,
+    sync::Arc,
     task::{Context, Poll},
 };
 
 use futures::future::{FutureExt, TryFutureExt};
+use tokio::sync::oneshot;
 use tower::{buffer::Buffer, util::BoxService, Service};
-use zebra_chain::parameters::Network;
+use zebra_chain::{
+    block::{self, Block},
+    parameters::Network,
+};
 
 use crate::{BoxError, Config, HashOrHeight, MemoryState, Request, Response, SledState};
 
+// todo: put this somewhere
+pub struct QueuedBlock {
+    pub block: Arc<Block>,
+    // TODO: add these parameters when we can compute anchors.
+    // sprout_anchor: sprout::tree::Root,
+    // sapling_anchor: sapling::tree::Root,
+    pub rsp_tx: oneshot::Sender<Result<block::Hash, BoxError>>,
+}
+
 struct StateService {
     /// Holds data relating to finalized chain state.
     sled: SledState,
@@ -39,12 +53,18 @@ impl Service<Request> for StateService {
         match req {
             Request::CommitBlock { block } => unimplemented!(),
             Request::CommitFinalizedBlock { block } => {
-                let rsp = self
-                    .sled
-                    .commit_finalized(block)
-                    .map(|hash| Response::Committed(hash));
+                let (rsp_tx, rsp_rx) = oneshot::channel();
 
-                async move { rsp }.boxed()
+                self.sled.queue(QueuedBlock { block, rsp_tx });
+                self.sled.process_queue();
+
+                async move {
+                    rsp_rx
+                        .await
+                        .expect("sender oneshot is not dropped")
+                        .map(|hash| Response::Committed(hash))
+                }
+                .boxed()
             }
             Request::Depth(hash) => {
                 // todo: handle in memory and sled
diff --git a/zebra-state/src/sled_state.rs b/zebra-state/src/sled_state.rs
index fb2cb510..848b4f45 100644
--- a/zebra-state/src/sled_state.rs
+++ b/zebra-state/src/sled_state.rs
@@ -1,16 +1,19 @@
 //! The primary implementation of the `zebra_state::Service` built upon sled
-use crate::Config;
-use std::{convert::TryInto, future::Future, sync::Arc};
+
+use std::{collections::HashMap, convert::TryInto, future::Future};
+
 use zebra_chain::serialization::ZcashSerialize;
 use zebra_chain::{
-    block::{self, Block},
+    block::{self},
     parameters::Network,
 };
 
-use crate::BoxError;
+use crate::{BoxError, Config, QueuedBlock};
 
-#[derive(Clone)]
 pub struct SledState {
+    /// Queued blocks that arrived out of order, indexed by their parent block hash.
+    queued_by_prev_hash: HashMap<block::Hash, QueuedBlock>,
+
     hash_by_height: sled::Tree,
     height_by_hash: sled::Tree,
     block_by_height: sled::Tree,
@@ -27,6 +30,7 @@ impl SledState {
         let db = config.sled_config(network).open().unwrap();
 
         Self {
+            queued_by_prev_hash: HashMap::new(),
             hash_by_height: db.open_tree(b"hash_by_height").unwrap(),
             height_by_hash: db.open_tree(b"height_by_hash").unwrap(),
             block_by_height: db.open_tree(b"block_by_height").unwrap(),
@@ -37,9 +41,33 @@ impl SledState {
         }
     }
 
+    /// Queue a finalized block to be committed to the state.
+    pub fn queue(&mut self, queued_block: QueuedBlock) {
+        let prev_hash = queued_block.block.header.previous_block_hash;
+        self.queued_by_prev_hash.insert(prev_hash, queued_block);
+    }
+
+    pub fn process_queue(&mut self) {
+        // Cloning means the closure doesn't hold a borrow of &self,
+        // conflicting with mutable access in the loop below.
+        let hash_by_height = self.hash_by_height.clone();
+        let tip_hash = || {
+            read_tip(&hash_by_height)
+                .expect("inability to look up tip is unrecoverable")
+                .map(|(_height, hash)| hash)
+                .unwrap_or(block::Hash([0; 32]))
+        };
+
+        while let Some(queued_block) = self.queued_by_prev_hash.remove(&tip_hash()) {
+            self.commit_finalized(queued_block)
+        }
+    }
+
     /// Commit a finalized block to the state. It's the caller's responsibility
     /// to ensure that blocks are committed in order.
-    pub fn commit_finalized(&self, block: Arc<Block>) -> Result<block::Hash, BoxError> {
+    fn commit_finalized(&mut self, queued_block: QueuedBlock) {
+        let QueuedBlock { block, rsp_tx } = queued_block;
+
         // The only valid block without a coinbase height is the genesis
         // block.  By this point the block has been validated, so if
         // there's no coinbase height, it must be the genesis block.
@@ -48,7 +76,7 @@ impl SledState {
         let hash = block.hash();
 
         use sled::Transactional;
-        (
+        let transaction_result = (
             &self.hash_by_height,
             &self.height_by_hash,
             &self.block_by_height,
@@ -68,9 +96,9 @@ impl SledState {
 
                 // for some reason type inference fails here
                 Ok::<_, sled::transaction::ConflictableTransactionError>(())
-            })?;
+            });
 
-        Ok(hash)
+        let _ = rsp_tx.send(transaction_result.map(|_| hash).map_err(Into::into));
     }
 
     // TODO: this impl works only during checkpointing, it needs to be rewritten
@@ -101,20 +129,7 @@ impl SledState {
         &self,
     ) -> impl Future<Output = Result<Option<(block::Height, block::Hash)>, BoxError>> {
         let hash_by_height = self.hash_by_height.clone();
-        async move {
-            Ok(hash_by_height
-                .iter()
-                .rev()
-                .next()
-                .transpose()?
-                .map(|(height_bytes, hash_bytes)| {
-                    let height = block::Height(u32::from_be_bytes(
-                        height_bytes.as_ref().try_into().unwrap(),
-                    ));
-                    let hash = block::Hash(hash_bytes.as_ref().try_into().unwrap());
-                    (height, hash)
-                }))
-        }
+        async move { read_tip(&hash_by_height) }
     }
 
     pub fn depth(&self, hash: block::Hash) -> impl Future<Output = Result<Option<u32>, BoxError>> {
@@ -137,3 +152,19 @@ impl SledState {
         }
     }
 }
+
+// Split into a helper function to be called synchronously or asynchronously.
+fn read_tip(hash_by_height: &sled::Tree) -> Result<Option<(block::Height, block::Hash)>, BoxError> {
+    Ok(hash_by_height
+        .iter()
+        .rev()
+        .next()
+        .transpose()?
+        .map(|(height_bytes, hash_bytes)| {
+            let height = block::Height(u32::from_be_bytes(
+                height_bytes.as_ref().try_into().unwrap(),
+            ));
+            let hash = block::Hash(hash_bytes.as_ref().try_into().unwrap());
+            (height, hash)
+        }))
+}