The commonest cause of pulmonary embolism is -

Thrombi in a deep vein of the leg accounts for more than 95% of cases of pulmonary embolism. The most common source of emboli is DVT of femoropopliteal and iliac veins. But remember that : Most common site of DVT is calf veins (Calf veins thrombosis are itself associated with a low risk for embolism, but when the thrombosis progresses proximally to involve the larger veins above knee, then the risk becomes very high.