The genealogies of 533 individuals with an intracranial aneurysm (IA) born in the Saguenay-Lac-Saint-Jean region, a geographically isolated area located in northeastern Quebec, were reconstructed using a population-based register. A control group consisting of three individuals of the same sex and born on the same day and in the same municipality than the IA patients was created; the genealogies of the 1599 controls were also reconstructed. The coefficients of inbreeding and kinship were calculated. Familial aggregation, i.e. the presence of IA in two or more first- to third-degree relatives, was also sought. The mean inbreeding coefficient was lower in the IA group than in the control group (7.92 x 10(-4) versus 10.04 x 10(-4)). The mean kinship coefficient was higher in the IA group than in the control group (2.17 x 10(-4) versus 1.55 x 10(-4)). Forty-eight IA patients (9.0%) were first-degree relatives compared to only 1.9% of the control individuals. The proportion of individuals showing familial aggregation was higher in the IA group than in the control group (29.8% and 18.6% respectively). These results strongly suggest that some IA are genetically determined in this population.