Patients with colitis and ileocolitis of unknown etiology from two previously reported series have been combined and the follow-up studies have been extended to compare the long term postoperative courses of ulcerative colitis (UC) and Crohn's disease of the colon (CDC). The combined and updated series of 176 patients, 99% of whom could be traced, provided a mean postoperative follow-up period for UC of 14 years (5 to 31) and CDC of 13.1 years (5 to 36). There were highly significant associations between generally accepted clinical and distributional features of UC and CDC and microscopic findings generally regarded as reliable for each. However, because of spectrum of features was found in each entity, neither clinical and distributional nor microscopic features alone are sufficient for diagnosis in every case. There were no differences in gross or disease-related mortality in UC and CDC whatever the method of diagnosis. After anastomotic procedures in CDC a recurrence rate of 73% was found. After proctocolectomy the ileostomy revision rate (considering all types of those which required further excision of ileum) was higher in CDC than UC whether the diagnoses were based on microscopic, clinical, or combined criteria, but the differences reached statistical significance only in the comparison of "clinical UC", with "clinical CDC". Moreover, after the first 2 postoperative years, the risk of having an ileostomy revision in UC and CDC (combined criteria) per patient year follow-up was virtually identical and there were no cases of short bowel syndrome. Differences in the clinical courses of UC and CDC after colectomy and ileostomy are of degree and do not reflect the ultimate course or potential for rehabilitation. Decisions regarding surgical therapy should be made independent of the diagnosis of UC or CDC.