Among a cohort of 981 children who were followed up 4.3-26.5 years after cessation of antileukemic therapy, eight patients in remission of acute lymphoblastic leukemia (ALL) developed a distinctively new malignant disease. The second malignant neoplasms (SMN) included brain tumors, basal cell carcinomas, thyroid cancer, leiomyosarcoma and finally rhabdomyosarcoma in a patient who also had suffered from Hodgkin's disease while still on antileukemic treatment. Cranial radiation had been given to 58.4% of the patients in the study group, which consisted of 895 ALL patients who had completed various chemotherapy protocols. With one exception, the SMN appeared after 7.5-16.5 years at a location previously exposed to radiotherapy (RT). The estimated cumulative risk of SMN appearing within 20 years after diagnosis was 2.9%, and the corresponding risk for cases with RT was 8.1% compared to 0.3% for those without (p = 0.05). In a Cox regression analysis, the incidence rate ratio of SMN between patients with and without RT was 6.7 (95% CI = 0.8, 57.7). Based on age-, year- and sex-specific cancer incidence figures for Norway, the overall standardized incidence rate ratio (SIR) of SMN after treatment for ALL was 5.9 (95% CI = 2.2, 12.9). The number of brain tumors among patients who had received cranial radiation was nearly 27 times greater than expected, whereas no such tumors were seen after chemotherapy. Individuals treated for childhood ALL are at increased risk of a new malignancy, and this seems mainly to be associated with previous irradiation.