diff --git a/README.md b/README.md
index 1b5a4807..3903e278 100644
--- a/README.md
+++ b/README.md
@@ -179,7 +179,18 @@ Stub mode does not support:
 
 # Known Issues
 
-* None
+* The synchronous lookup functions will not work when new file descriptors
+  needed for the lookup will be larger than `FD_SETSIZE`.  This is because
+  the synchronous functions use a "default" event loop under the hood
+  which is based on `select()` and thus inherits the limits that `select()` has.
+
+  If you need only slightly more file descriptors, it is possible to enlarge
+  the `FD_SETSIZE` with the `--with-fd-setsize=`*`size`* flag to `configure`.
+
+  To resolve, use the asynchronous functions with an event loop extension for
+  libevent, libev or libuv.  Note that the asynchronous functions will have
+  the same problem when used in combination with `getdns_context_run()`, which
+  also uses the default event loop.
 
 # Supported Platforms