diff --git a/ae_epoll.c b/ae_epoll.c
index f6371b0be..ce9ce3b01 100644
--- a/ae_epoll.c
+++ b/ae_epoll.c
@@ -29,10 +29,13 @@ static void aeApiFree(aeEventLoop *eventLoop) {
 static int aeApiAddEvent(aeEventLoop *eventLoop, int fd, int mask) {
     aeApiState *state = eventLoop->apidata;
     struct epoll_event ee;
+    /* If the fd was already monitored for some event, we need a MOD
+     * operation. Otherwise we need an ADD operation. */
     int op = eventLoop->events[fd].mask == AE_NONE ?
             EPOLL_CTL_ADD : EPOLL_CTL_MOD;
 
     ee.events = 0;
+    mask |= eventLoop->events[fd].mask; /* Merge old events */
     if (mask & AE_READABLE) ee.events |= EPOLLIN;
     if (mask & AE_WRITABLE) ee.events |= EPOLLOUT;
     if (mask & AE_EXCEPTION) ee.events |= EPOLLPRI;