Among 161 necropsy cases of aortic dissection, 87 (54%) were type I, 34 (21%) type II, and 40 (25%) type III, and an intimal tear was identified in each. Systemic hypertension had been present in 63 of 121 cases (52%) with type I or II dissection and in 30 of 40 (75%) with type III dissection. Aortic dissection involved 7 of 16 cases (44%) with the Marfan syndrome. In the 154 cases without the Marfan syndrome, grade 3 or 4 medial degeneration (cystic medial necrosis) was observed in the ascending aorta in only 27 (18%). The risk of aortic dissection in persons with congenitally bicuspid and unicommissural aortic valves, respectively, was 9 and 18 times that in subjects with tricuspid aortic valves. The mean age of those with aortic dissection and tricuspid, bicuspid and unicommissural aortic valves was 63, 55 and 40 years, respectively, and aortic dissection was more common in men than in women. Grade 3 or 4 atherosclerosis involved the intimal tear in only 11 of 121 type I or II dissections (9%) but 32 of 40 type III dissections (80%). Accordingly, the major risk factors for aortic dissection were systemic hypertension, the Marfan syndrome, and, for type I and II dissections, congenitally bicuspid or unicommissural aortic valves. Aortic medial degeneration was a less important risk factor. Rupture of ulcerocalcific aortic atheromas may have initiated the intimal tear in some type III dissections.