Less commonly, strokes are caused by the rupture of a weak spot, called an aneurysm, on a blood vessel in the brain. When the aneurysm ruptures, blood flow is interrupted. If the blood vessel is on the surface of the brain (subarachnoid hemorrhaging), the blood leaks into the fluid surrounding the brain. If the vessel is within the brain (cerebral hemorrhaging), the blood accumulates within the brain. In addition to injury caused by loss of circulation, the pooled blood can damage brain cells by putting pressure on them.

There are three primary causes of strokes. Thrombosis and embolism both cut off the brain's blood supply, leading to tissue death. Rupture of a blood vessel in or near the brain is the cause of hemorrhage. Since any part of the brain can be affected by a stroke, symptoms vary considerably. Thrombosis is blockage by a clot (thrombus ) that has gradually built up on the wall of a brain artery. Cerebral thrombosis causes 225,000 strokes a year. Embolism is the blocking of an artery in the brain by a clot (embolus ) that has moved through the blood system to the brain. Embolism annually accounts for 190,000 strokes. Hemorrhage results from the rupture of a blood vessel and subsequent bleeding within the brain. Approximately 85,000 strokes a year are caused by hemorrhage.