To assess the effectiveness of azathioprine and 6-mercaptopurine in inducing remission of active Crohn disease and the effectiveness of azathioprine in maintaining remission of quiescent disease.

Pertinent studies were selected from the MEDLINE database (1966 to May 1994), abstracts from major gastrointestinal meetings, and references from published articles and reviews.

Nine randomized, placebo-controlled trials of azathioprine or 6-mercaptopurine therapy were identified: Four addressed active disease, two addressed quiescent disease, and three had multiple therapeutic arms.

Data were extracted by three independent observers on the basis of the intention-to-treat principle and were analyzed with logistic regression. Each study was given a quality score on the basis of predetermined criteria.

Compared with placebo, azathioprine or 6-mercaptopurine therapy had an odds ratio of response of 3.09 (95% CI, 2.45 to 3.91) in patients with active Crohn disease. When the single trial that used 6-mercaptopurine in active disease was excluded from the analysis, the odds ratio of response was 1.45 (CI, 1.12 to 1.87). No trials of quiescent disease used 6-mercaptopurine; the odds ratio of response in these trials of quiescent disease was 2.27 (CI, 1.76 to 2.93). For active disease, continuation of therapy for at least 17 weeks improved response (P = 0.03). For quiescent disease, a higher dose improved response (P = 0.008). Increased cumulative dose improved response in both groups (P < 0.001 for active disease and P = 0.01 for quiescent disease). A steroid-sparing effect was seen in active disease (odds ratio, 3.69 [CI, 2.12 to 6.42] and in quiescent disease (odds ratio, 4.64 [CI, 1.00 to 21.54]). Fistulae improved with therapy (odds ratio, 4.44 [CI, 1.50 to 13.20]). Adverse events requiring withdrawal from a trial, primarily allergy, leukopenia, pancreatitis, and nausea, were increased with therapy (odds ratio, 5.26 [CI, 2.20 to 12.60]).

Azathioprine and 6-mercaptopurine are effective in treating active Crohn disease and in maintaining remission. Cumulative dose was an important factor in predicting response. Adverse effects were more common among patients receiving therapy.