Spontaneous internal fistulas involving intestine, rectum, bladder, or vagina in patients without predisposing illnesses were studied. Twelve of 20 (60 percent) were receiving steroids or other anti-inflammatory medication at the time the fistulas developed. Fifteen of the 20 patients subsequently had diverticular disease identified. It is proposed that there is an association between anti-inflammatory medication and fistulas that develop from intestine to other pelvic viscera, spontaneously or in association with colonic diverticula.