There is doubt about the timing of surgery for patients with internal fistulas in Crohn's disease. Although immediate operative intervention for all patients has been advocated, such a policy has not always been followed at St. Mark's Hospital. Between 1971 and 1982, 83 internal fistulas were identified in 59 patients with Crohn's disease. Fifty-nine fistulas arose primarily from the small intestine and involved another segment of the bowel, five were between large bowel and duodenum, and three between areas of large bowel. Sixteen fistulas (ten from ileum and six from large bowel) involved the bladder. Thirty-six patients with 54 fistulas underwent immediate surgical treatment. Fifteen patients with 20 fistulas required surgery later. There was one postoperative death among the 51 patients treated surgically and one late death unrelated to the treatment of the fistula. Of the remaining 49 surgically treated patients, 46 were traced and remain well, six after further surgery. Eight patients with nine fistulas (four involving the bladder) were treated at St. Mark's without operation. One later required surgery elsewhere for an enterocutaneous fistula, but the remaining seven patients are well. This study suggests that the presence of an internal fistula, even if it involves the bladder, is not an absolute indication for immediate surgery and that the severity of the symptoms should dictate the treatment policy.