Ventilation-perfusion lung scans were done in 70 patients admitted because of an iliofemoral deep vein thrombosis (DVT), and diagnosed by venography. Eighteen items of clinical and laboratory information were measured to predict the presence of lung scan abnormalities consistent with asymptomatic pulmonary embolism. Eighteen patients had perfusion defects in the lung scan that were classified as "high probability of pulmonary embolism", while the lung scan was normal in 32 patients. Patients with an idiopathic DVT were at high risk of pulmonary embolism, but those with a postoperative DVT had a low embolic risk. In patients with a postoperative DVT two factors predicted pulmonary embolism: those patients who developed symptomatic DVT after an operation but had a normal lung scan showed higher platelet counts than their preoperative levels. Conversely, it they developed a pulmonary embolism their platelet count dropped from their preoperative levels.