Currently, approximately 67% of children diagnosed with cancer can be expected to survive more than 5 years. Among the most significant late effects of cancer therapy is the development of second malignant neoplasm (SMN). This study was performed to identify the factors associated with the development of second malignant neoplasms after treatment for soft tissue sarcomas in childhood. Retrospectively the charts of 20 children who developed second malignant neoplasms after treatment for primary childhood soft tissue sarcoma were reviewed. Presentation, age at diagnosis, tumor histology, extent of tumor, treatment, family histories (when available), and outcome were recorded. The mean age of the patients (10 boys, 10 girls) was 8.5 years of age (range, 1 to 20 years). Most primary tumors were rhabdomyosarcoma (14/20) and occurred in an extremity (10/20). Ninety percent of the patients (18/20) had a complete response to treatment of the primary cancer. Eleven out of 20 received combined chemotherapy and radiation therapy. The most common secondary malignancy was a bone sarcoma (6/20), followed by brain tumors (n = 3), leukemia (n = 2), and other sarcomas (n = 2). Four of the bone sarcomas developed in the field of radiation treatment. Median follow-up was 16 years (range, 1 to 26 years). The median time to development of a SMN was 11.4 years (range, 1.5 to 21 years). Survival after a second malignancy was only 30%. Two patients developed a third malignant neoplasm. The occurrence of a secondary malignancy represents a serious complication of childhood cancer. Certain tumors are related directly to treatment such as osteosarcoma within irradiated fields and secondary leukemias or lymphomas after certain chemotherapy regimens. Combined radiotherapy and chemotherapy may play an additive role in the development of second malignant neoplasms. Genetic factors may predispose affected patients to the development of both primary and secondary malignancies. Close surveillance of children previously treated for childhood cancers is warranted.