Myositis ossificans (MO) is a benign, localized, ossifying lesion of soft tissues which may be mistaken for extraskeletal osteosarcoma. Usual locations include the musculature of the upper arm, upper leg, buttocks and hands. The occurrence of this lesion in young children is rarely reported, with the bulk of childhood bone-forming lesions of soft tissue being due to MO progressiva. Following a recent case, a 28 yr search of surgical pathology files from 1964-91 was conducted to determine the prevalence of MO amongst biopsies received at the AMCWC. Only 1 further case was found. The clinicopathological features are detailed below: 1) A 7yo boy presented with a non-tender anterior chest wall mass that had been painlessly enlarging over 6 weeks. Radiological investigations demonstrated a soft tissue lesion with focal ossification of uncertain nature. Histologic examination of the excised 20x35x40mm mass revealed a zonal pattern typical of maturing MO with a central loosely cellular region composed of spindled ceils surrounded by a rim of well formed woven bone trabeculae lined by osteoblasts. 2) A 7.5 yo boy presented with a tender right quadriceps mass that had been enlarging over 3 wks. Histologic examination of a small wedge biopsy revealed loose fibroblastic stroma with foci of osteoblasts surrounding osteoid and woven bone; compatible with the early stages of MO. No significant atypia was present. Within 12 days of presentation X-rays showed the typical features of MO; and followup over 3yrs showed no evidence of further growth or malignancy. There were no other lesions or digital malformations present in either patient to suggest MO progressiva. Neither patient had a history of trauma. These patients demonstrate that MO should be considered in the differential diagnosis of ossifying soft tissue masses in young children. Both radiological and pathological diagnosis may be difficult, particularly in early lesions where the characteristic zoning pattern has not developed. Although MO progressiva always should be excluded, the most significant differential diagnoses are extraosseous and juxtacortical osteosarcomas. Myositis ossificans (MO) is a benign, localized, ossifying lesion of soft tissues which may be mistaken for extraskeletal osteosarcoma. Usual locations include the musculature of the upper arm, upper leg, buttocks and hands. The occurrence of this lesion in young children is rarely reported, with the bulk of childhood bone-forming lesions of soft tissue being due to MO progressiva. Following a recent case, a 28 yr search of surgical pathology files from 1964-91 was conducted to determine the prevalence of MO amongst biopsies received at the AMCWC. Only 1 further case was found. The clinicopathological features are detailed below: 1) A 7yo boy presented with a non-tender anterior chest wall mass that had been painlessly enlarging over 6 weeks. Radiological investigations demonstrated a soft tissue lesion with focal ossification of uncertain nature. Histologic examination of the excised 20x35x40mm mass revealed a zonal pattern typical of maturing MO with a central loosely cellular region composed of spindled ceils surrounded by a rim of well formed woven bone trabeculae lined by osteoblasts. 2) A 7.5 yo boy presented with a tender right quadriceps mass that had been enlarging over 3 wks. Histologic examination of a small wedge biopsy revealed loose fibroblastic stroma with foci of osteoblasts surrounding osteoid and woven bone; compatible with the early stages of MO. No significant atypia was present. Within 12 days of presentation X-rays showed the typical features of MO; and followup over 3yrs showed no evidence of further growth or malignancy. There were no other lesions or digital malformations present in either patient to suggest MO progressiva. Neither patient had a history of trauma. These patients demonstrate that MO should be considered in the differential diagnosis of ossifying soft tissue masses in young children. Both radiological and pathological diagnosis may be difficult, particularly in early lesions where the characteristic zoning pattern has not developed. Although MO progressiva always should be excluded, the most significant differential diagnoses are extraosseous and juxtacortical osteosarcomas.