diff --git a/doc/environment_variables.md b/doc/environment_variables.md
index 47efb3a1d874db1864d44082d834ec67c8380e43..dce434ff30d6985a655c3aebadbb9e2ecdc7fe47 100644
--- a/doc/environment_variables.md
+++ b/doc/environment_variables.md
@@ -4,6 +4,10 @@ gRPC environment variables
 gRPC C core based implementations (those contained in this repository) expose
 some configuration as environment variables that can be set.
 
+* http_proxy
+  The URI of the proxy to use for HTTP CONNECT support.  Does not currently
+  support username or password information in the URI.
+
 * GRPC_ABORT_ON_LEAKS
   A debugging aid to cause a call to abort() when gRPC objects are leaked past
   grpc_shutdown(). Set to 1 to cause the abort, if unset or 0 it does not