Periosteal sunburst spiculation is a peculiar radiographic feature of osteosarcoma, and it represents a reactive ossification resulting from the action of normal osteoblasts rather than tumor cells. Because bone morphogenetic protein is known to be a potent inducer of ectopic bone formation, the authors hypothesized that bone morphogenetic protein may be involved in the pathogenesis of such reactive bone formation in osteosarcoma. Chinese hamster ovary cells transfected with the bone morphogenetic protein-4 gene were injected into the femurs of athymic nude mice to form experimental bone tumors producing bone morphogenetic protein. Two weeks after intramedullary injection, new bone formation was observed radiographically and histologically within the extraosseous portions of the tumors. This showed a close resemblance to sunburst spiculation in human osteosarcomas. In contrast, the control nontransformed Chinese hamster ovary tumors showed no extraosseous bone formation. Because the induced bone was composed of multiple parallel spicules similar to those found in human bone morphogenetic protein-producing osteosarcomas, these findings suggest that periosteal sunburst spiculation may be the result of bone morphogenetic protein production by osteosarcoma cells.