ABSTRACT: A 6-year-old girl developed transient diabetes mellitus while receiving L-asparaginase, prednisone, vincristine, 6-mercaptopurine, and methotrexate as chemotherapy for acute lymphocytic leukemia that went into remission after 3 years of therapy. The patient subsequently developed autoimmune thyroid disease with hypothyroidism. Islet-cell antiboides were detected 1 year after anti-leukemic chemotherapy was stopped and 5 years before she developed overt insulin-dependent diabetes mellitus (IDDM). It is suggested that the immunosuppressive drugs received during anti-leukemic therapy may have delayed the apparance of IDDM and thyroid autoimmunity in a genetically susceptible patient.