The use of azathioprine in ulcerative colitis is unclear. The authors present the details and outcome of 47 patients who received azathioprine for either a) severe, resistant disease otherwise requiring surgery (28 patients) or b) patients with steroid dependence who have been followed up for at least 12 months (19 patients). Duration of treatment ranged from one week to 66 months (median, 12 months). Of the patients in Group I, 13 (46 percent) achieved remission, 11 of whom had not relapsed during a median follow-up of 22 months (range, 12 to 58 months), and 15 underwent surgery one week to 12 months (median, five weeks) after commencing azathioprine. In Group II, steroids were withdrawn or reduced in 12 (63 percent) patients and three patients required colectomy. Side effects necessitating withdrawal of azathioprine occurred in 12 patients (hematologic effects, 6 patients; gastrointestinal effects, 4 patients; other effects, 2 patients). Two patients required a reduced dose of azathioprine because of leukopenia. The authors conclude that azathioprine is a valuable therapeutic option in selected patients with ulcerative colitis.