This article reviews current chemotherapy of childhood acute lymphoblastic leukemia, with particular emphasis on the pharmacology of the drugs used. In the perspective of the overall treatment plan, the use, mode of action, toxicity and pharmacology of prednisone, vincristine, L-asparaginase, cyclophosphamide, 6-mercaptopurine, methotrexate and cytosine arabinoside are reviewed. Issues relating to central nervous system prophylaxis, drug compliance, drug resistance, and treatment failure are considered.