The alkaloid vincristine was administered intravenously once each week to 25 children with advanced cancer beginning with doses of 3 mg per estimated square meter of body surface. Objective tumor response of 1 to 5 mo duration was noted in 17 of 25 children in whom 9 of the 10 tumor types studied were represented. Complete remission was achieved in nine cases. There were six cases of acute lymphocytic leukemia, one of Hodgkin's disease, one of lymphosarcoma, and one of Wilms's tumor. Six other children experienced more than 50% tumor reduction and two children had 25% to 50% tumor reduction. Toxic manifestations included alopecia (17), absent patellar and Achilles tendon reflexes (14), leukopenia (12), vomiting (9), constipation (9), oral ulceration (3), hypertension (6), slapping gait (3), abducens paralysis (3), and visual defects (2).