Thrombin is present in the lungs of patients with primary extremity osteosarcoma and pulmonary metastases. Osteosarcoma is a rare cancer, which metastasizes to the lung in up to 80% of cases. Thrombin is involved in metastasis and is present in the lungs of patients with pulmonary metastases (PM). To identify its role in PM and osteosarcoma, we measured thrombin levels in the bronchoalveolar lavage fluid (BALF) of 15 patients. BALF was collected at different stages of the disease and correlated with the diagnosis of PM. We also assessed fibrinogen overexpression in the tumors. We found that 11/15 (73%) patients with high thrombin levels in the lungs developed PM within the first 12 months from primary surgery. The median thrombin concentration in the BALF of these patients increased up to 8x10(-9) M (range, 3x10(-9)M-15x10(-9)M), which represents a more than 100-fold increase compared to patients without PM (p<0.0001). Eight of 15 (53%) primary and 11/15 (73%) metastatic samples showed fibrinogen overexpression. A significant difference between high thrombin levels, fibrinogen overexpression and PM was found compared to patients without PM (p=0.00073 and p=0.025). These results show that thrombin levels are increased in the lungs of patients with primary osteosarcoma and a high risk of developing PM. They suggest that thrombin may be involved in the development of PM.