Bone scintigraphy was performed in 17 patients with previously known lung metastases of osteosarcoma. 99Tcm-MDP uptake was observed in all primary bone lesions but lung metastatic lesions were positive in only six patients (35%). 99Tcm-MDP uptake by lung metastases was significantly correlated with bone and osteoid formation in the metastatic lesions and preoperative serum ALPase values. These clinical observations were confirmed by using nude mice transplanted with human lung metastatic osteosarcoma. 99Tcm-MDP scintigraphy appears to be useful for detecting lung metastases of osteosarcoma only in a selected group of patients.