We used intermediate doses of Ara-C (IDAra-C) in the treatment of 15 patients with chronic myelogenous leukemia (CML) in blast crisis and, combined with hydroxyurea, in 20 CML patients in accelerated phase. Patients with blastic CML received intensive 5-day courses of IDAra-C 600 mg/m2 every 12 h as a 2-h infusion. Of 15 patients, three achieved complete response (CR) and three partial response (PR), for an overall response rate of40 per cent. All patients developed severe leukopenia and thrombocytopenia, and two died in hypoplasia. Except nausea and vomiting requiring medication, other nonhematologic toxicities were uncommon. Median response duration was 4 months (range 1 to 7 months). Survival was 5 months for responders and 1.5 months for nonresponders. Patients with CML in accelerated phase were treated with two-day courses of IDAra-C 600mg/m2 every 12 h by 2-h infusion, every two-three weeks. Daily hydroxyurea 1-1.5 g/day was administered between courses. Of 20 patients, 15 (75 per cent) achieved a good PR with rapid improvement of the symptoms of disease acceleration. The median duration of response was 11 months (range 3 to 38 months); duration was over 24 months in five patients. The median survival from the start of IDAra-C was 13 months for responders and 3.5 months for nonresponders. We conclude that IDAra-C is an effective approach for CML in terminal phase. Its use in 5-day induction courses for blast crisis CML has a response rate comparable to that achieved with high-dose Ara-C. In patients in accelerated phase, the combination of short courses of IDAra-C with hydroxyurea is a well-tolerated treatment able to improve substantially the clinical and hematologic symptoms of disease progression.