diff --git a/src/client_server.rs b/src/client_server.rs
index 0536c8e..4a2f33b 100644
--- a/src/client_server.rs
+++ b/src/client_server.rs
@@ -890,7 +890,7 @@ pub fn upload_keys_route(
     if let Some(one_time_keys) = &body.one_time_keys {
         for (key_key, key_value) in one_time_keys {
             db.users
-                .add_one_time_key(sender_id, device_id, key_key, key_value)?;
+                .add_one_time_key(sender_id, device_id, key_key, key_value, &db.globals)?;
         }
     }
 
@@ -1002,7 +1002,7 @@ pub fn claim_keys_route(
         for (device_id, key_algorithm) in map {
             if let Some(one_time_keys) =
                 db.users
-                    .take_one_time_key(user_id, device_id, key_algorithm)?
+                    .take_one_time_key(user_id, device_id, key_algorithm, &db.globals)?
             {
                 let mut c = BTreeMap::new();
                 c.insert(one_time_keys.0, one_time_keys.1);
@@ -2912,7 +2912,11 @@ pub async fn sync_events_route(
             changed: device_list_updates.into_iter().collect(),
             left: Vec::new(), // TODO
         },
-        device_one_time_keys_count: Default::default(), // TODO
+        device_one_time_keys_count: if db.users.last_one_time_keys_update(sender_id)? > since {
+            db.users.count_one_time_keys(sender_id, device_id)?
+        } else {
+            BTreeMap::new()
+        },
         to_device: sync_events::ToDevice {
             events: db.users.get_to_device_events(sender_id, device_id)?,
         },
diff --git a/src/database.rs b/src/database.rs
index a837638..5a1ed0f 100644
--- a/src/database.rs
+++ b/src/database.rs
@@ -75,6 +75,7 @@ impl Database {
                 userdeviceid_metadata: db.open_tree("userdeviceid_metadata")?,
                 token_userdeviceid: db.open_tree("token_userdeviceid")?,
                 onetimekeyid_onetimekeys: db.open_tree("onetimekeyid_onetimekeys")?,
+                userid_lastonetimekeyupdate: db.open_tree("userid_lastonetimekeyupdate")?,
                 keychangeid_userid: db.open_tree("devicekeychangeid_userid")?,
                 keyid_key: db.open_tree("keyid_key")?,
                 userid_masterkeyid: db.open_tree("userid_masterkeyid")?,
diff --git a/src/database/users.rs b/src/database/users.rs
index 7fb9767..c792767 100644
--- a/src/database/users.rs
+++ b/src/database/users.rs
@@ -22,6 +22,7 @@ pub struct Users {
     pub(super) token_userdeviceid: sled::Tree,
 
     pub(super) onetimekeyid_onetimekeys: sled::Tree, // OneTimeKeyId = UserId + AlgorithmAndDeviceId
+    pub(super) userid_lastonetimekeyupdate: sled::Tree, // LastOneTimeKeyUpdate = Count
     pub(super) keychangeid_userid: sled::Tree,       // KeyChangeId = RoomId + Count
     pub(super) keyid_key: sled::Tree,                // KeyId = UserId + KeyId (depends on key type)
     pub(super) userid_masterkeyid: sled::Tree,
@@ -270,6 +271,7 @@ impl Users {
         device_id: &DeviceId,
         one_time_key_key: &AlgorithmAndDeviceId,
         one_time_key_value: &OneTimeKey,
+        globals: &super::globals::Globals,
     ) -> Result<()> {
         let mut key = user_id.to_string().as_bytes().to_vec();
         key.push(0xff);
@@ -294,14 +296,32 @@ impl Users {
                 .expect("OneTimeKey::to_string always works"),
         )?;
 
+        self.userid_lastonetimekeyupdate.insert(
+            &user_id.to_string().as_bytes(),
+            &globals.next_count()?.to_be_bytes(),
+        )?;
+
         Ok(())
     }
 
+    pub fn last_one_time_keys_update(&self, user_id: &UserId) -> Result<u64> {
+        self
+            .userid_lastonetimekeyupdate
+            .get(&user_id.to_string().as_bytes())?
+            .map(|bytes| {
+                utils::u64_from_bytes(&bytes).map_err(|_| {
+                    Error::bad_database("Count in roomid_lastroomactiveupdate is invalid.")
+                })
+            })
+            .unwrap_or(Ok(0))
+    }
+
     pub fn take_one_time_key(
         &self,
         user_id: &UserId,
         device_id: &DeviceId,
         key_algorithm: &KeyAlgorithm,
+        globals: &super::globals::Globals,
     ) -> Result<Option<(AlgorithmAndDeviceId, OneTimeKey)>> {
         let mut prefix = user_id.to_string().as_bytes().to_vec();
         prefix.push(0xff);
@@ -311,6 +331,11 @@ impl Users {
         prefix.extend_from_slice(key_algorithm.to_string().as_bytes());
         prefix.push(b':');
 
+        self.userid_lastonetimekeyupdate.insert(
+            &user_id.to_string().as_bytes(),
+            &globals.next_count()?.to_be_bytes(),
+        )?;
+
         self.onetimekeyid_onetimekeys
             .scan_prefix(&prefix)
             .next()