BACKGROUND: Nineteen patients with Ewing sarcoma of bone, each younger than 3 years of age at the time of initial presentation, are reported. These children represent an unusually young age group for documented Ewing sarcoma and represent 2.6% of all patients registered in the Intergroup Ewing's Sarcoma Study (IESS). METHODS: The diagnosis was made on combined histologic, clinical, and radiographic grounds and was supported by immunohistochemistry in nine patients and electron microscopic study in eight patients. Immunohistochemical studies showed no staining against neuro-specific enolase, chromogranin, or S-100 in any patients, and in only one in nine patients were results of such studies focally positive against Leu-7. RESULTS: The most marked clinical variation between this group of infants and the general IESS patient population was a striking predominance of female patients in the younger age group (P < 0.001). There was a trend toward more rib, pelvis, and proximal long bone tumors in the infants and toddlers. The overall survival rate of the infants was 56%, almost identical to the survival rates of the older children. All of the infants who died of disease did so within 4 years. Extended follow-up as long as 9.9 years has found no late deaths attributable to tumor. The most significant late complications of therapy included cardiotoxicity with cerebral embolization from the heart and a short lower limb secondary to radiation therapy. CONCLUSIONS: Ewing sarcoma must be considered in the differential diagnosis of small cell, round cell, and blue cell tumors in bone, even in infants and toddlers.