Four cases of synovial sarcoma with extensive calcification and osteoid and bone formation are reported. Ages ranged from 21 to 38 years. Two tumors were located in the foot and two in the thigh. Because of a well-circumscribed, densely calcified soft tissue mass, radiologically three patients were thought to have a benign lesion. The fourth patient was thought to have a paraosteal osteosarcoma because of an accompanying bone defect. Tumor size varied from 4.0 to 9.0 cm. Histologically, three tumors were biphasic and one predominantly monophasic. All showed amorphous calcifications with extensive ossification sometimes in a ribbon-like pattern of osteoid, simulating osteosarcoma. The extensive bone formation with abundant osteoid deposition may lead to a misdiagnosis of osteosarcoma. It is important to recognize this variant of synovial sarcoma with ossification and bone formation and distinguish it from extraskeletal osteosarcoma because of the difference in clinical behavior and course. Although the most important point in the recognition of this variant of synovial sarcoma is its biphasic pattern, this may not be apparent in a small tissue sample. Points that aid in the diagnosis include the uniform nuclear appearance of both the epithelial and the spindle cells versus the pleomorphism of osteosarcoma and in some cases the presence of amorphous concretions in sheets and small calcospherites within spaces surrounded by flat or conspicuous epithelial cells. These cells are immunoreactive for cytokeratin and epithelial membrane antigen.