We evaluated, retrospectively, the outcome of 56 patients (39 male, 17 female; mean age, 34 years; age range, 14-65 years) who received azathioprine for either steroid-resistant (group A, n = 10) or steroid-dependent (group B, n = 46) ulcerative colitis. The patients were followed for a mean of 29 +/- 17 months (median, 27 months). Twenty-four had left-sided colitis, 5 had subtotal colitis, and 27 had total colitis. The mean duration of the disease was 51 months (range, 2-192 months). At the beginning of azathioprine treatment (time 0), all patients had clinically severe disease and were taking 40 mg prednisolone per day. Azathioprine was used in addition to steroid therapy at a dosage of 2 mg/kg. The need for steroids, expressed as the median cumulative steroid dose (mg/year), and the number of clinical relapses (requiring steroid therapy) in the 2 years before azathioprine treatment, were compared with those of the 3-year follow-up with azathioprine treatment. A positive response to azathioprine was defined as (a) avoidance of colectomy, (b) a significant decrease in the median cumulative steroid dose, and (c) a significant decrease in the number of clinical relapses (expressed as number/patient/year). One patient in group A withdrew due to painful dyspepsia, and two patients in group B were lost to follow-up. Remission with complete elimination of steroids was achieved in 36 of 53 (64%), 23 of 35 (66%), and 18 of 26 (69%) patients in the first, second, and third years, respectively, of azathioprine treatment. Compared with the 2 years before azathioprine treatment, a significant decrease was observed of about 75% both in steroid consumption and in the number of clinical relapses during the 3 years of azathioprine therapy. Two of nine patients in group A and 2 of 44 patients in group B had colectomy after mean periods of 15 months and 24 months, respectively. Azathioprine is effective and safe in avoiding colectomy in patients with steroid-resistant and steroid-dependent ulcerative colitis; its use decreases both steroid requirements and clinical relapses.