Causes. A stroke occurs when the blood supply to your brain is interrupted or reduced. This deprives your brain of oxygen and nutrients, which can cause your brain cells to die. A stroke may be caused by a blocked artery (ischemic stroke) or the leaking or bursting of a blood vessel (hemorrhagic stroke). Thrombotic stroke. 2  A thrombotic stroke occurs when a blood clot (thrombus) forms in one of the arteries that supply blood to your brain. 3  A clot may be caused by fatty deposits (plaque) that build up in arteries and cause reduced blood flow (atherosclerosis) or other artery conditions. 4  Embolic stroke.