A case of osteosarcoma with unusual microscopic features, occurring in the right proximal tibial metaphysis of a 12-year-old boy is reported. Radiographically, the tumor was ill-defined and purely osteolytic. On gross examination, the tumor was soft, fragile, spongy and red to brown in color. Microscopically, the tumor consisted of pleomorphic cells possessing abundant eosinophilic cytoplasm, including cells larger than 100 microm in diameter. The cells were arranged in a sheet, with few extracellular collagen fibers. Multiple sectioning of the specimens revealed a small amount of osteoid production. Immunohistochemical study revealed a positive reaction for vimentin and osteocalcin. Electron microscopic study suggested the fibroblastic or osteoblastic origin of the cells.