Abdominal aortic aneurysm: results of a family study. Data pertaining to abdominal aortic aneurysm among first-degree relatives of 91 patients with abdominal aortic aneurysm are presented. The percentage of families with at least one affected first-degree relative of the proband (multiplex families) was 15.4%. In 21.4% of multiplex families parent-offspring transmission of abdominal aortic aneurysm was noted; in the remaining families only siblings were affected. The mean age at onset among probands was 67.3 years; that among all affected was 67.4 years. No statistically significant difference in the mean ages at onset between genders was noted. Among affected siblings of probands, the sex ratio, male:female, was 1.33:1, which is not significantly different from 1:1. The relative risk of developing an abdominal aortic aneurysm was 3.97 for fathers, 4.03 for mothers, 9.92 for brothers, and 22.93 for sisters. 