Fistula formation between a segment of colon and the uterus is an unusual complication of diverticulitis; only 17 cases have been reported in the world literature. We describe a 69-year-old woman with a colouterine fistula secondary to diverticulitis. She presented with a malodorous vaginal discharge that grew multiple enteric organisms on culture. A barium enema revealed colonic diverticula but no fistula tract. Orally administered activated charcoal was seen flowing from the cervical os during a pelvic examination the following day, establishing a diagnosis of colouterine fistula. Pathologic examination of the resected colon and uterus confirmed this diagnosis and determined that diverticulitis was the etiology. From a review of the literature, we conclude that radiographic and invasive procedures cannot be depended upon for diagnosis. Ingestion of activated charcoal may provide a simple, noninvasive approach to the diagnosis of enterouterine fistulas.