diff --git a/synapse/handlers/sliding_sync.py b/synapse/handlers/sliding_sync.py
index e2fc578ac8..ccee85b63b 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -192,7 +192,7 @@ class SlidingSyncHandler:
 
         if timeout == 0 or from_token is None:
             now_token = self.event_sources.get_current_token()
-            return await self.current_sync_for_user(
+            result = await self.current_sync_for_user(
                 sync_config,
                 from_token=from_token,
                 to_token=now_token,
@@ -215,6 +215,8 @@ class SlidingSyncHandler:
                 from_token=from_token,
             )
 
+        return result
+
     async def current_sync_for_user(
         self,
         sync_config: SlidingSyncConfig,
@@ -249,11 +251,11 @@ class SlidingSyncHandler:
             ops: List[SlidingSyncResult.SlidingWindowList.Operation] = []
             for range in list_config.ranges:
                 ops.append(
-                    {
-                        "op": OperationType.SYNC,
-                        "range": range,
-                        "room_ids": sorted_room_ids[range[0] : range[1]],
-                    }
+                    SlidingSyncResult.SlidingWindowList.Operation(
+                        op=OperationType.SYNC,
+                        range=range,
+                        room_ids=sorted_room_ids[range[0] : range[1]],
+                    )
                 )
 
             lists[list_key] = SlidingSyncResult.SlidingWindowList(
@@ -426,7 +428,7 @@ class SlidingSyncHandler:
             # `to_token`.
             if (
                 last_membership_change_after_to_token.membership == Membership.LEAVE
-                and prev_membership != None
+                and prev_membership is not None
                 and prev_membership != Membership.LEAVE
             ):
                 sync_room_id_set.add(room_id)
@@ -438,7 +440,7 @@ class SlidingSyncHandler:
             # the room before the `to_token`.
             elif (
                 last_membership_change_after_to_token.membership != Membership.LEAVE
-                and (prev_membership == None or prev_membership == Membership.LEAVE)
+                and (prev_membership is None or prev_membership == Membership.LEAVE)
             ):
                 sync_room_id_set.discard(room_id)
 
diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index 92bde70011..a08b8df388 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -33,7 +33,7 @@ from synapse.events.utils import (
     format_event_raw,
 )
 from synapse.handlers.presence import format_user_presence_state
-from synapse.handlers.sliding_sync import SlidingSyncConfig
+from synapse.handlers.sliding_sync import SlidingSyncConfig, SlidingSyncResult
 from synapse.handlers.sync import (
     ArchivedSyncResult,
     InvitedSyncResult,
@@ -810,7 +810,44 @@ class SlidingSyncRestServlet(RestServlet):
 
         logger.info("sliding_sync_results: %s", sliding_sync_results)
 
-        return 200, {"foo": "bar"}
+        response_content = await self.encode_response(sliding_sync_results)
+
+        return 200, response_content
+
+    # TODO: Is there a better way to encode things?
+    async def encode_response(
+        self,
+        sliding_sync_result: SlidingSyncResult,
+    ) -> JsonDict:
+        response: JsonDict = defaultdict(dict)
+
+        response["next_pos"] = await sliding_sync_result.next_pos.to_string(self.store)
+        response["lists"] = self.encode_lists(sliding_sync_result.lists)
+        response["rooms"] = {}  # TODO: sliding_sync_result.rooms
+        response["extensions"] = {}  # TODO: sliding_sync_result.extensions
+
+        return response
+
+    def encode_lists(
+        self, lists: Dict[str, SlidingSyncResult.SlidingWindowList]
+    ) -> JsonDict:
+        def encode_operation(
+            operation: SlidingSyncResult.SlidingWindowList.Operation,
+        ) -> JsonDict:
+            return {
+                "op": operation.op,
+                "range": operation.range,
+                "room_ids": operation.room_ids,
+            }
+
+        serialized_lists = {}
+        for list_key, list_result in lists.items():
+            serialized_lists[list_key] = {
+                "count": list_result.count,
+                "ops": [encode_operation(op) for op in list_result.ops],
+            }
+
+        return serialized_lists
 
 
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None: