The role of aortic diameter on the occurrence of type A dissection was investigated in 73 patients with dilated ascending aorta at the time of pre-operative evaluation. Using transthoracic echocardiography for diagnosis and measurements, 54 patients were identified with type A dissection (group 1) and 19 without dissection (group 2). The true mean aortic diameters were identical (6.0 +/- 1.3 cm in group 1 and 6.4 +/- 1.4 cm in group 2; mean +/- SD; ns) as were the indexed aortic diameters (ratio of diameter/body surface area; 3.2 +/- 0.8 cm.m-2 and 3.4 +/- 0.7 cm.m-2, respectively; ns). However, the individual diameters showed a pronounced scatter in both groups (range from 3.6 +/- 11.0 cm). Of the 73 patients, 66 had surgery (47/54 with and 19/19 without dissection) and seven patients were treated medically. Emergency surgery was performed in 45/66 patients (all with acute type A dissection) and elective repair in 21/66 (19 without and two with chronic type A dissection). In-hospital mortality was 18% in the emergency group, 5% in the elective group and 57% in the medical group. It is concluded that patients with dilated ascending aorta have a substantial incidence of acute dissection. Their clinical course is unpredictable: acute dissection occurs in some, and in others the ascending aorta continues to enlarge without dissection. Because patients with dissection often arrive too late for elective repair and have to be operated on as emergencies with a higher operative risk, we recommend elective surgery before the diameter of the ascending aorta has reached 6 cm.