The commonest site of thrombosis is -

Venous thrombi (phlebothrombosis) frequently propagate some distance toward the hea, forming a long cast within the vessel lumen that is prone to give rise to emboli. An increase in the activity of coagulation factors is involved in the genesis of most venous thrombi, with platelet activation playing a secondary role. Contain more enmeshed red cells, leading to the moniker red, or stasis, thrombi. The veins of the lower extremities are most commonly affected (90% of venous thromboses); however, venous thrombi also can occur in the upper extremities, periprostatic plexus, or ovarian and periuterine veins, and under special circumstances may be found in the dural sinuses, poal vein, or hepatic vein. ( Robbins Basic Pathology, 9 th edition, page 88 )