Forty-six patients who underwent surgery for brain tumors were studied prospectively with 125I labeled Fibrinogen leg scans to detect postoperative venous thrombosis. The incidence of thrombosis was 72% for meningioma patients, 60% for glioblastoma patients, and 20% for brain metastasis patients. Correlation between the occurrence of venous thrombosis and the various clinical factors thought to be responsible for the high incidence of thrombosis generally failed to show statistical significance. This finding, along with the marked variation in the incidence of venous thrombosis between the different brain tumor groups, strongly suggests that biological factors play a more important role than clinical factors in determining which brain tumor patient will suffer a postoperative thrombotic event.