Embolism from the heart causes about one-fifth of ischemic strokes and transient ischemic attacks. Although a potential embolic source may be found in up to one-third of cases, which are actually caused by other mechanisms, in the young patient more than one source of embolism is less likely. The most-important embolic sources are non-rheumatic and rheumatic atrial fibrillation (AF), infective endocarditis, prosthetic heart valves, recent myocardial infarction, dilated cardiomyopathy, intracardiac tumors, and rheumatic mitral stenosis. Uncomplicated mitral valve prolapse should no longer be considered a cause of embolism from the heart to the brain; an additional disorder (i.e., gross mitral regurgitation, AF, infective endocarditis) must be present to suggest a cardioembolic origin of the stroke.