Two children with Ewing's sarcoma developed acute nonlymphocytic leukemia (ANLL) during the course of their illness. One patient developed ANLL after apparently successful treatment of his primary malignancy with radiation therapy and multiagent chemotherapy. In the second patient, acute leukemia developed before the administration of radiotherapy or systemic chemotherapy. The development of secondary ANLL after Ewing's sarcoma has been reported only twice previously, most likely representing a therapy-induced complication. The occurrence of ANLL in Patient 2 prior to therapy suggests that these two disorders may have a more than treatment-related association. Close follow-up of long-term survivors of Ewing's sarcoma with surveillance for secondary acute leukemia is advised.