Background & Aims A small placebo-controlled trial reported the efficacy of mercaptopurine therapy for children newly diagnosed with Crohn's disease, yet little is known about the efficacy of early thiopurine therapy in adults. Methods We performed a prospective double-blind trial of adult patients with a recent (<8 weeks) diagnosis of Crohn's disease. Patients were randomly assigned to groups given azathioprine (2.5 mg · kg−1 · day−1, n = 68) or placebo (n = 63) at 31 hospitals from February 2006 to September 2009. Corticosteroids but no other concomitant medications were allowed for control of disease activity. The primary measure of efficacy was sustained corticosteroid-free remission. Results After 76 weeks of treatment, 30 patients treated with azathioprine (44.1%) and 23 given placebo (36.5%) were in sustained corticosteroid-free remission (difference of 7.6%; 95% confidence interval, −9.2 to 24.4%; P = .48). The rates of relapse (defined as Crohn's Disease Activity Index score >175) and corticosteroid requirements were similar between groups. A post hoc analysis of relapse, defined as a Crohn's Disease Activity Index score >220, showed lower relapse rates in the azathioprine group than in the placebo group (11.8% vs 30.2%; P = .01). Serious adverse events occurred in 14 patients in the azathioprine group (20.6%) and 7 in the placebo group (11.1%) (P = .16). A larger percentage of patients in the azathioprine group had adverse events that led to study drug discontinuation (20.6%) than in the placebo group (6.35%) (P = .02). Conclusions In a study of adults with Crohn's disease, early azathioprine therapy was no more effective than placebo to achieve sustained corticosteroid-free remission but was more effective in preventing moderate to severe relapse in a post hoc analysis. EudraCT 2005-001186-34.