Oral 6-mercaptopurine (6-MP) is an integral component of maintenance chemotherapy for children with acute lymphoblastic leukemia (ALL). A clinical comparative study of 6-MP given at 175 mg/m2/day vs. 250 mg/m2/day for 5 days was carried out. Ten patients on a CCLSG ALL 874 and 911 study received 6-MP at 175 mg/m2/day and 9 patients on a CCLSG ALL 941 study received 6-MP at 250 mg/m2/day. There was no significant difference in the value of WBC, Hb, and platelet counts between these two groups. The mean value of GPT level was significantly increased in a higher dosage group with no delay of 6-MP administration. The mean value of GPT peak level was decreased gradually following repeated cycles of 6-MP administration. Five days of 6-MP at 250 mg/m2/day was well tolerated and safe for the maintenance chemotherapy of childhood ALL.