Fistula between the large bowel and an ovarian cyst is a recognised but rare pathological entity. We present two cases of colo-ovarian cyst fistula, the first occurring as a result of diverticular disease of the sigmoid colon and the second complicating a cystadenocarcinoma of the ovary. The aetiology of colo-ovarian cyst fistula is reviewed and the differential diagnosis is discussed.