A 9-year prospective study of 542 consecutive patients undergoing operation by one of the authors for abdominal aortic aneurysms was undertaken to define the incidence, clinical behavior, and anatomic characteristics of familial abdominal aortic aneurysms. Eighty-two (15.1%) patients having surgery for abdominal aortic aneurysms were found to have a first-degree relative with an aneurysm, as compared to nine (1.8%) of a control group of 500 patients of similar age and sex without aneurysmal disease (p less than 0.001). Detailed analysis was next performed of the pedigree charts of patients with a positive family history of aneurysm who underwent repair of abdominal aortic aneurysms by all authors over the 9-year study period. This review identified a total study population of 86 families with 209 first-degree relatives with abdominal aortic aneurysms. Clinical and anatomic features of this familial group were compared to those of 460 patients operated on for abdominal aortic aneurysms who had no family history of abdominal aortic aneurysms. Patients with familial abdominal aortic aneurysms were more likely to be women (35% vs 14%), and men with familial abdominal aortic aneurysms tended to be about 5 years younger than the women. There was no significant difference between the patients with nonfamilial and familial abdominal aortic aneurysms in anatomic extent of aneurysmal disease, multiplicity of aneurysms, associated occlusive disease, or blood type. There was a history of aneurysm rupture in 35 of 86 (40.7%) families with familial abdominal aortic aneurysms.(ABSTRACT TRUNCATED AT 250 WORDS)