Arterial thrombi are frequently occlusive; the most common sites in decreasing order of frequency are the coronary, cerebral, and femoral arteries.They typically consist of a friable meshwork of platelets, fibrin, red cells, and degenerating leukocytes.Although these are usually superimposed on a ruptured atherosclerotic plaque, other vascular injuries (vasculitis, trauma) may be the underlying cause.Under special circumstances, they can also occur in the dural sinuses, portal vein, or hepatic vein.Venous thrombi are firm, are focally attached to thevessel wall, and contain lines of Zahn, features that help distinguish them from postmortem clots (see later).The veins of the lower extremities are most commonly involved (90% of cases); however, upper extremities, periprostatic plexus, or the ovarian and periuterine veins can also develop venous thrombi.Venous thrombosis (phlebothrombosis) is almost invari- ably occlusive, with the thrombus forming a long luminal cast.Postmortem clots can sometimes be mistaken for ante- mortem venous thrombi.