diff --git a/src/fontra_rcjk/client_async.py b/src/fontra_rcjk/client_async.py
index f0d09df..998155c 100644
--- a/src/fontra_rcjk/client_async.py
+++ b/src/fontra_rcjk/client_async.py
@@ -1,10 +1,46 @@
 import asyncio
+import logging
+from collections import defaultdict
+from contextlib import asynccontextmanager
 
 import aiohttp
 
 from .client import Client as RCJKClient
 from .client import HTTPError
 
+logger = logging.getLogger(__name__)
+
+MAX_CONCURRENT_CALLS = 140  # MySQL's default max connections is 151
+
+
+class ConcurrentCallLimiter:
+    def __init__(self):
+        self.num_calls_in_progress = 0
+        self.event_queue = []
+
+    @asynccontextmanager
+    async def limit(self):
+        if self.num_calls_in_progress >= MAX_CONCURRENT_CALLS:
+            if not self.event_queue:
+                logger.info("limiting concurrent API calls")
+            event = asyncio.Event()
+            self.event_queue.append(event)
+            await event.wait()
+
+        self.num_calls_in_progress += 1
+        try:
+            yield
+        finally:
+            self.num_calls_in_progress -= 1
+            if self.num_calls_in_progress < MAX_CONCURRENT_CALLS and self.event_queue:
+                event = self.event_queue.pop(0)
+                event.set()
+                if not self.event_queue:
+                    logger.info("done limiting concurrent API calls")
+
+
+call_limiters = defaultdict(ConcurrentCallLimiter)
+
 
 class RCJKClientAsync(RCJKClient):
     def _connect(self):
@@ -13,6 +49,7 @@ def _connect(self):
         pass
 
     async def connect(self):
+        self._call_limiter = call_limiters[self._host]
         self._session = aiohttp.ClientSession(connector=aiohttp.TCPConnector(ssl=False))
         session = await self._session.__aenter__()
         assert session is self._session
@@ -52,6 +89,11 @@ async def get_project_font_uid_mapping(self):
         return project_font_uid_mapping
 
     async def _api_call(self, view_name, params=None):
+        async with self._call_limiter.limit():
+            result = await self._api_call_unlimited(view_name, params)
+        return result
+
+    async def _api_call_unlimited(self, view_name, params=None):
         url, data, headers = self._prepare_request(view_name, params)
         async with self._session.post(url, data=data, headers=headers) as response:
             if response.status == 401: