Estimates of familial aggregation of migraine have varied considerably due, in part, to methodological differences among studies. We concluded a population-based study of 73 clinically confirmed probands with migraine, 72 matched control probands, and 511 of their first-degree relatives, all of whom were directly interviewed. The risk of migraine was 50% more likely in relatives of migraine probands than in relatives of controls. Migraine risk was considerably higher among relatives of probands with disabling migraine compared with relatives of probands with minimal disability. Moreover, for probands with minimal disability, no excess risk of migraine in female relatives was observed. Finally, in relatives of male migraine probands, there appears to be an excess risk of migraine with aura. A borderline significant relative risk of 4.04 was observed. No excess risk was observed among relatives of male probands who had migraine without aura. This study suggests that familial factors (environment related to the family or genetic factors) account for less than one-half of all migraine cases in the population. Degree of disability in the proband appears to influence familial risk. These results suggest that the development of migraine is determined by complex genetic as well as environmental factors.