Nineteen patients (12 children and 7 adults) with severe Crohn's disease, all of whom were dependent on corticosteroids, were treated with 6-mercaptopurine. All patients received a daily dose of 6-mercaptopurine of 50 mg; in two pediatric patients with a poor response after 2 months, the dosage was increased to 75 mg/day. A complete or partial response to 6-mercaptopurine therapy was noted in 47% of patients, and therapy failed in 53%. The age of the patients, prior resection, or initial symptoms did not influence the response. The clinical response was better in male than in female patients and in patients with involvement of both the small intestine and the colon than in those with only enteritis. 6-Mercaptopurine is a possible alternative to long-term corticosteroid therapy or surgical treatment in selected patients with severe Crohn's disease.