BACKGROUND & AIMS: 6-mercaptopurine/azathioprine is effective in IBD patients. However, data regarding toxicity associated with pregnancy are lacking, raising both patients' and physicians' concerns and sometimes resulting in elective abortion. METHODS: To evaluate potential toxicity of 6-mercaptopurine (6-MP), we reviewed the records of 485 patients who had received the drug. We contacted 462, of whom 155 had conceived at least 1 pregnancy after developing IBD. Pregnancies were analyzed as to whether the patient had taken 6-MP before, or at the time of, conception. These were compared with IBD patients who had their pregnancies before taking 6-MP. We collected data on live births, spontaneous abortions, prematurity, abortions secondary to birth defects, major and minor congenital birth defects, infections, and neoplasia. Outcomes were analyzed comparing pregnancies from men and women who had taken or were currently taking 6-MP to controls. RESULTS: There was no statistical difference in conception failures (defined as a spontaneous abortion), abortion secondary to a birth defect, major congenital malformations, neoplasia, or increased infections among male or female patients taking 6-MP compared with controls (RR = 0.85 [0.47-1.55], P = 0.59). CONCLUSIONS: 6-MP use before or at conception or during pregnancy appears to be safe. Discontinuation of the drug before and during pregnancy is not indicated.