BACKGROUND The occurrence of second malignant neoplasms (SMNs) in successfully treated pediatric patients with cancer has been an area of increasing concern because survival of these patients has improved with intensification of therapy. Therefore, the incidence of SMNs in long term survivors of childhood rhabdomyosarcoma (RMS) was studied. METHODS From 1970 to 1989, 210 newly diagnosed patients (median age, 9.7 years; range, 1 month to 27 years) with RMS were treated at Memorial Sloan-Kettering Cancer Center (New York, NY). Multimodality treatment included chemotherapy, surgery, and radiotherapy, when indicated. There were 130 long term survivors (> 2 years off therapy) with a median follow-up of 9 years (range, 2-20 years). The cumulative dose of each chemotherapeutic agent and the radiation doses each patient received were reviewed. Statistical analysis was performed by comparison with the Connecticut Tumor Registry data. RESULTS Seven patients developed a SMN, including three with acute nonlymphoblastic leukemia (ANLL) and four with solid tumors. Acute nonlymphoblastic leukemia developed a median of 4.5 years after diagnosis. Of the solid tumors, 3 developed within the radiation field at a median of 10 years after diagnosis, whereas the fourth occurred 9.3 years after initial diagnosis in a patient who did not receive radiotherapy. All seven patients with SMNs received total dactinomycin doses higher than the median (9.6 mg/M2) for the group. All three patients with ANLL received total cyclophosphamide doses higher than the median (16.8 g/M2). Moreover, six of the seven patients received a dose of radiotherapy greater than 4000 cGy. The standardized incidence ratio was: 17.07 (95% confidence interval, 6.68-35.18; P < 0.0001). CONCLUSIONS Multimodality therapy has improved long term survival for patients with childhood RMS. The combination of high dose radiotherapy and chemotherapy appears to increase the risk for developing a second malignancy.