To determine the efficacy of azathioprine in the treatment of ulcerative colitis, a 6-month double blind trial was carried out. Thirty patients with chronic ulcerative colitis who required the equivalent of at least 10 mg of prednisone per day over the 3 months prior to entering the study were randomized into placebo and azathioprine (1.5 mg per kg) treatment groups. Reduction of steriods was a major objective of the trial. Age and sex distribution, number of bowel movements, sense of well being, steroid dosage, and findings on proctoscopy, rectal biopsy, and colon X-ray initially were similar in the two groups. No side effects were associated with azathioprine. Although steroid dose was lower (p less than 0.05) in the azathioprine group at the termination of the study, no difference between the two groups could be detected in the number of bowel movements, sense of well being, and findings on proctoscopy during the first 3 weeks compared with the last 3 and during the first 3 months compared with the last 3. Although azathioprine does not confer dramatic benefit upon patients with chronic ulcerative colitis who require steroids, it does permit reduction of steroid dosage without apparent worsening of the disease. Its major value in ulcerative colitis may be in facilitating significant decreases or complete discontinuance of steroids.