A phase II trial of prolonged IV infusions of vincristine was conducted in 21 patients with refractory acute leukemia. Patients received 0.25-0.50 mg/m2 by infusion daily for 5 days after an initial 0.5-mg bolus. A partial response was observed in one of two patients with acute lymphoblastic leukemia. Of 14 patients with acute nonlymphoblastic leukemia, a complete response lasting for 2.5 months occurred in one patient and a partial response lasting 1.3 months was observed in a second. No objective responses were noted in five patients with blast crisis of chronic granulocytic leukemia. Nonhematologic toxicity was minimal and, when present, generally consisted of a feeling of weakness; constipation, mucositis, and areflexia were also observed. Hematologic toxicity consisted mainly of mild to moderate reduction of platelets in most patients; marked thrombocytopenia (less than 50,000/mm3) occurred in two patients whose pretreatment platelet count was greater than 100,000/mm3. Although generally well tolerated, prolonged infusion of vincristine appears to have limited activity in the treatment of refractory acute nonlymphoblastic leukemia and blast crisis of chronic granulocytic leukemia; further evaluation is needed in acute lymphoblastic leukemia refractory to conventional bolus injection.