Six patients (two females and four males in one family) with hemiplegic migraine are described. The age of onset was between six and eighteen years. Hemiplegic attacks were usually accompanied by transient neurological disturbances referable to the territory of the vertebrobasilar arterial tree. Brain-stem involvement and vertebrobasilar dysfunction in the reported family was supported by angiography in one case, revealing a marked and prolonged spasm of the basilar artery. Despite its dramatic character and occasionally prolonged deficits the course was essentially benign. Hemiplegic attacks disappeared during adolescence in five of the six patients. No permanent neurological residual phenomena were encountered during a mean follow-up of 14 years. The possibility is raised of a genetically determined susceptibility to periodic vasoconstriction in a particular vascular area as the basis of the syndrome.