Effective forms of treatment for acute lymphoblastic leukemia (ALL) in childhood now result in survival rates above 70 percent at five years, but the treatments are potentially carcinogenic. To determine the magnitude of this risk and identify possible risk factors for the development of second neoplasms, we studied a large cohort of children treated for ALL. We undertook a retrospective cohort study of 9720 children who had been given a diagnosis of ALL between June 1972 and August 1988 and had been treated according to the therapeutic protocols of the Children's Cancer Study Group. The median follow-up was 4.7 years (range, 2 months to 16 years). We found that 43 second neoplasms occurred among the children in the cohort, including 24 neoplasms of the central nervous system, 10 new leukemias and lymphomas, and 9 other neoplasms. This represented a 7-fold excess of all cancers and a 22-fold excess of neoplasms of the central nervous system. The estimated cumulative proportion of children in whom a second neoplasm developed was 2.53 percent 15 years after diagnosis (95 percent confidence limits, 1.74 percent and 3.38 percent). An even higher risk, particularly of central nervous system tumors, was evident in children five years of age or less at the time of the diagnosis of ALL (P = 0.012). All central nervous system neoplasms developed in children who had previously undergone irradiation. There was no association with exposure to cyclophosphamide or anthracyclines. There is a substantial excess of second neoplasms, especially of the central nervous system, among children treated for ALL. Children five years old or younger and those receiving radiation are at higher risk, especially for second tumors arising in the central nervous system.