Summary A survey was made of the clinical manifestations and postoperative course of 48 patients who had been subjected to total or partial colectomy, for the inflammatory reaction of which manifested varying degrees of resemblance to Crohn's disease. Criteria of selection for the survey were histological and not clinical. Cases in which the disease was believed to be associated with extensive regional ileitis were excluded. Histologically, the cases showed a spectrum between a typical Crohn's type of inflammation and a picture resembling ulcerative colitis with serosal involvement. Clinically, the cases fell into two groups, those which resembled ulcerative colitis (16 cases) and those which resembled Crohn's disease of the large bowel (32 cases). There was a tendency for cases to conform to their histological picture but the number of exceptions made prognostication difficult. Fistulae tended to persist postoperatively for longer periods than in classical ulcerative colitis, but once postoperative fistulae had healed in these disorders no further fistulae formation occurred. A limited resection of localized disease where performed proved to be adequate. A surprising high incidence of rectal carcinoma was observed (three out of five patients followed up for more than 8 years). This occurred exclusively in the cases which presented the appearances of histologically atypical ulcerative colitis. From this study, it is concluded that the term primary Crohn's disease of the colon is probably not an entirely adequate designation for all patients with this histological picture, since many patients with this histological spectrum are clinically indistinguishable from ulcerative colitis. Subsequent small intestinal recurrence did not occur in our series, and thus the prognosis of Crohn's disease of the colon is inarkedly different from that of regional ileitis.