To investigate the incidence and outcome of secondary neoplasms in pediatric patients treated for childhood cancer. Between December 1971 and January 2000, a total of 5859 patients younger than age 17 were diagnosed and treated for childhood cancers in our center. Of this group, 1511 (36%) patients were followed for more than 36 months. These long-term survivors were included in this analysis. Twenty-six patients developed a secondary malignancy with an overall risk of 1.7% in this cohort. The male:female ratio was 17:10, with a median age of 7.66 at diagnosis (range, 2 to 16 y). Four patients (14.8%) with Hodgkin lymphoma; 3 each (11.1%) with retinoblastoma and rhabdomyosarcoma; 2 each (7.4%) with Wilms tumor, Ewing sarcoma, medulloblastoma, ganglioneuroblastoma, and non-Hodgkin lymphoma; and 1 each (3.7%) with ependymoma, nasopharyngeal carcinoma, osteosarcoma, astrocytoma had a secondary malignant disease during the long-term follow-up period. Secondary malignant diseases were osteosarcoma in 6 patients, acute lymphoblastic leukemia in 2, acute myelogenous leukemia in 2, and rare malignant disease in others. Four patients with osteosarcoma developed disease within the radiation field. Osteosarcoma was the most frequently occurring secondary neoplasm. Less toxic treatment modalities should be used to decrease the risk of secondary malignant diseases.