Familial hemiplegic migraine is an autosomal dominant disorder of unknown pathogenesis in which the migrainous attacks are marked by the occurrence of a transient hemiplegia during the aura. While investigating CADASIL, mapped previously to chromosome 19, we observed that some patients had recurrent attacks of migraine with aura. Although the clinical and neuroimaging features of familial hemiplegic migraine differ markedly from CADASIL, we hypothesized that the same gene could be involved in the pathogenesis of both conditions. We chose two large pedigrees for linkage analysis of familial hemiplegic migraine. A maximum lod score > 8 was found with two markers that are also strongly linked to CADASIL. Multilocus linkage analysis suggested that the loci responsible for the two diseases reside within an interval of about 30 cM on chromosome 19.