6-Thioguanine (6-TG) was administered as a continuous i.v. infusion for 7 days to 24 patients with relapsed or refractory acute leukemia or in the blast phase of chronic granulocytic leukemia. The daily dose of 6-TG was escalated from 37.5 mg/m2 to 160 mg/m2. Stomatitis was dose-related and dose-limiting with a maximum tolerated dose of 120 mg/m2 daily. Cutaneous reactions were dose-related but not dose-limiting. The recommended dose for phase II trials in acute leukemia is 120 mg/m2 per day as a continuous infusion for 7 days. There were two complete and four partial remissions among all patients. At the suggested phase II dose of 120 mg/m2 there were two complete remissions and one partial remission in five evaluable patients.