BACKGROUND & AIMS: Because the reoperation rate for Crohn's disease is high after resective surgery, use of conservative surgery has increased. Mesalamine was investigated for the prevention of postoperative relapse, with disappointing results. The role of azathioprine in the postoperative setting is unknown. We aimed to compare the efficacy and safety of azathioprine and mesalamine in the prevention of clinical and surgical relapse in patients who have undergone conservative surgery for Crohn's disease. METHODS: In a prospective, open-label, randomized study, 142 patients received azathioprine (2 mg. kg -1. day -1 ) or mesalamine (3 g/day) for 24 months. Clinical relapse was defined as the presence of symptoms with a Crohn's Disease Activity Index score >200 and surgical relapse as the presence of symptoms refractory to medical treatment or complications requiring surgery. RESULTS: After 24 months, the risk of clinical relapse was comparable in the azathioprine and mesalamine groups, both on intention-to-treat (odds ratio [OR], 2.04; 95% confidence interval [CI], 0.89-4.67) and per-protocol analyses (OR, 1.79; 95% CI, 0.80-3.97). No difference was observed with respect to surgical relapse at 24 months between the 2 groups. In a subgroup analysis, azathioprine was more effective than mesalamine in preventing clinical relapse in patients with previous intestinal resections (OR, 4.83; 95% CI, 1.47-15.8). More patients receiving azathioprine withdrew from treatment due to adverse events than those receiving mesalamine (22% vs. 8%; P = 0.04). CONCLUSIONS: While no difference was observed in the efficacy of azathioprine and mesalamine in preventing clinical and surgical relapses after conservative surgery, azathioprine is more effective in those patients who have undergone previous intestinal resection.