The value of different diagnostic tests in the detection of colovesical fistulas was studied in 28 surgically proven cases seen during the last 10 years at the University of Michigan, Ann Arbor. Etiologies were diverticulitis (19), Crohn disease (three), postradiation therapy (four), previous trauma (one), and bladder carcinoma (one). The fistulas were demonstrated by barium enema in 10 of 20 patients and by cystography in eight of 26. Cystoscopy was diagnostic in 11 of 25 patients and sigmoidoscopy in four of 24. Methylene blue test was positive in five of six patients, and in one patient given a charcoal enema the material appeared in the urine. The Bourne test, consisting of radiography of the centrifuged urine samples obtained immediately after a nondiagnostic barium enema, was positive in nine of 10 patients. In seven of these 10 patients, the Bourne test was the only positive evidence of an otherwise occult colovesical fistula later proven at surgery.