Sixteen patients with accelerated or blastic phase chronic myelogenous leukemia were treated with high-dose cytosine arabinoside given at 3 g/m2 every 12 h for two to eight doses per course. All patients had an immediate clearing of peripheral blood blast cells. Five patients achieved a complete response for 1-14 months, median 4.5 months. Toxicity was significant and predictable on the basis of dose given. We have identified both an effective and a toxic dosing schedule. The rapid response is dramatic but relatively short-lived, and suggests that additional agents in conjunction with reinfusion of autologous stem cells may offer these patients a greater chance of significant response.