In patients presenting with clinically suspected deep vein thrombosis, symptomatic pulmonary embolism is rarely apparent. To assess the prevalence of silent pulmonary embolism in outpatients with proven deep vein thrombosis but without symptoms of pulmonary embolism, perfusion ventilation lung scans were performed in 101 consecutive patients at presentation. Fifty-one percent of these patients had a high probability lung scan at the initiation of treatment. In comparison, in patients referred with suspected venous thrombosis, but who on subsequent objective testing did not have venous thrombosis (n = 44), the prevalence of a high probability-scan for pulmonary embolus was only 5 percent. At repeat lung scanning, performed after one week of anticoagulant treatment, complete to partial improvement was observed in 68 percent of the patients with initially abnormal scans. Lung-scan detected asymptomatic pulmonary embolism occurs frequently in patients presenting with symptomatic deep venous thrombosis, and the majority of these emboli showed significant to complete resolution within one week of anticoagulant treatment.