During a five year period at the Harlem Hospital Center nine heroin addicts were seen with strokes. Four occurred after loss of consciousness following intravenous heroin. Two occurred in patients using heroin at the time, but were not related to overdose or to a particular recent injection. The youth of these patients and lack of other predisposing factors suggests that heroin played a role in their strokes. In the other three patients, the relationships of stroke to heroin is less persuasive. There are several possible mechanisms by which heroin abuse could lead to stroke.