We evaluated the efficacy of azathioprine (Az) therapy in chronic ulcerative colitis (CUC) retrospectively in a clinic population from 1977 to 1987. The study was an open one. Sixteen patients were treated with a total of 20 courses of Az. Mean dose of Az was 1.48 mg/kg/d (range 0.98-1.82). All patients were on oral prednisone at the initiation of Az therapy. Indications for Az were: inability to wean off steroids without symptoms of CUC (12/20), poor response to steroids (7/20), and steroid toxicity (1/20). Responses to Az occurred in 15/20 courses (12/16 patients). Two patients had side effects requiring Az discontinuation. The mean duration of follow-up was 52.6 months with no incidence of malignancies. We conclude that the use of Az in selected patients with CUC who are steroid resistant or steroid dependent may be both efficacious and safe.