Abstract: Methods of treating patients suffering from Crohn's disease or ulcerative colitis who did not experience a clinical response to previous thiopurine administration, or suffered side effects from previous thiopurine administration, by administering a delayed release pharmaceutical composition comprising 6-mercaptopurine are disclosed. Methods of treating patients suffering from Crohn's disease or ulcerative colitis who are also being administered a steroid, 5-aminosalicylic acid, or an antibiotic by adjunctively administering a delayed release pharmaceutical composition comprising 6-mercaptopurine are also disclosed.