Over the past 15 years (1974-1989), 126 patients with Crohn's disease received azathioprine (n = 123) or 6-mercaptopurine (n = 3). Seven patients were lost of follow-up during the first month and were not analyzed. Among the 119 patients analyzed (52 men, 67 women, mean age 31.6 years), the median duration of treatment was 9.1 months (range: 8 days to 15 years). Most of the 109 patients with active disease at the beginning of the immunosuppressive therapy were either steroid-dependent (n = 63) or steroid-resistant (n = 23); 19 had frequent relapses and/or extensive involvement of the gastrointestinal tract; 4 had severe perianal disease. Among these 109 patients, 25.4, 51, 60.4 and 64.4 percent were in clinical remission at 3, 6, 9 and 12 months (life-table analysis) respectively. There was no difference in outcome of patients with colonic, ileocolonic or small intestinal involvement. Steroid-resistant patients fared better than steroid-dependent patients (81 percent vs 59 percent of remission within the first year; p less than 0.001). In the 62 patients with quiescent disease treated with azathioprine or 6-mercaptopurine, previous remission had been achieved with immunosuppressive therapy in 52, 8 were treated after a bowel resection which was assumed to be curative; 2 had achieved remission after total parenteral nutrition. In these patients, the percentages of relapse were 15.3 at the end of the first year and 20.3 at the end of the second year. Among the 8 patients treated with azathioprine after bowel resection, only one relapse occurred 1.5 months after surgery. Nineteen percent of the patients had adverse reactions that required discontinuation of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)