Both azathioprine and its active metabolite, 6-mercaptopurine, are of benefit in the treatment of inflammatory bowel disease, either in resistant cases, or for their steroid-sparing effect. Azathioprine treatment is limited in some patients by hypersensitivity reactions or other side effects. We report our experience in 11 patients previously unable to tolerate azathioprine for a variety of reasons, who were switched to 6-mercaptopurine. Of seven patients with ulcerative colitis and four patients with Crohn's disease who were treated with 6-mercaptopurine following failed azathioprine therapy, six were able to successfully tolerate the substitute medication, with good response. Where patients have previously been intolerant of azathioprine yet ongoing indications for immunosuppressive therapy remain, a trial of 6-mercatopurine may be warranted. Given the similar efficacies of the two drugs in inflammatory bowel disease, these findings also favor the use of 6-mercaptopurine rather than the parent compound as initial therapy.