Alkaline phosphatase levels in osteosarcoma tissue are related to prognosis. Levels of alkaline phosphatase were measured in the primary tumor of 26 patients with osteosarcoma. One of seven patients with a tissue alkaline phosphatase level less than 0.6 microM/min/mg developed pulmonary metastases. In contrast, 16 or 17 patients with a tissue alkaline phosphatase level greater than 0.6 microM/min/mg developed pulmonary metastases. It thus appears that tissue alkaline phosphatase levels of primary osteosarcomas are strongly correlated with prognosis (p less than .01).