The long-term outcome of patients with a pathological diagnosis of indeterminate colitis on a colectomy specimen was investigated. The case records of 46 such patients operated on for inflammatory bowel disease between 1960 and 1983 were reviewed. Using the preoperative clinical information, pathological and radiological reports, it was possible to divide the patients into three groups: group 1, probable Crohn's disease (19 cases); group 2, probable ulcerative colitis (11 cases); and group 3, indeterminate colitis (16 cases). The patients were followed for a minimum of 2.5 years (median 10, range 2.5-28 years). During this period the probable diagnosis changed in five cases only. One patient in group 1 (Crohn's disease) was subsequently considered to have ulcerative colitis (group 2). The other four patients were all in group 3. Three were reclassified as ulcerative colitis and the fourth as Crohn's disease on the finding of a single granuloma in a rectal biopsy. No case in group 3 required subsequent small bowel surgery. These data suggest that patients continuing with a diagnosis of indeterminate colitis in spite of careful preoperative and postoperative assessment are unlikely to show features of Crohn's disease in the long term. This may be important when considering a subsequent restorative proctectomy.