I m assuming you mean someone had a stroke caused by a blood clot (ischemic stroke) as opposed to a bleeding stroke (heamorrhagic stroke)... Blood clots inside arteries are caused by a variety of things. Blood is constantly in balance between clotting itself and thinning itself. When the balance is met things are good. When it tips in eitehr direction, you can bleed out, or have clots form. Once clots form, the tend to get caught in the narrowest passageway. Depending on where that artery is, different symptoms can occur. If its the head you get a stroke, if its the foot you get blue toes etc... Clots can form on the right side or venous circulation (DVT). But for them to get to theleft side (systemic circulation) you need a hole in the heart to pass through. Otherwise they get filtered by the lung. Clots that form on the left side (arterial in origin) can be form a variety of reasons. Most common is atrial fibrillation, an irregular beating of the heart. Others can be from valvular diseases such as mitral stenosis or incompetent mitral valve prolapse. They can also occur if there isinfection of the heart (endocarditis). Clots can also form right at the site of blockage too. This is usually associated with diabetes, high blood pressure, cholesterol and smoking. Finally age is he biggest risk factor (over 70yrs). Hypercoagulability (propensity to clot more) can be caused by genetic factors (factor V deficiency, protein C or S deficiency), carrying the lupus anticoagulant gene (usually associated with repeated miscarriages). Certain drugs can cause this. Birth control pills or injections, combined with smoking is a bad combination. And the worst drug of all.... smoking. Usually this is the root of all things medical. Hope that helps.