To determine the incidence and mortality as well as to analyze the clinical and pathologic changes of aortic dissection. A population-based longitudinal study over 27 years on a study population of 106,500, including 66 hospitalized and 18 nonhospitalized consecutively observed patients. Analysis of data from the medical, surgical, and autopsy records of patients with aortic dissection. Altogether, 86 cases of aortic dissection were found in 84 patients, corresponding to a 2.9/100,000/yr incidence. Sixty-six of the 84 patients (79%) were admitted to the hospital, and 18 patients (21%) died before admission. Their ages ranged from 36 to 97 years, with a mean of 65. 7 years. The male/female ratio was 1.55 to 1. A total of 22.7% of the hospitalized patients died within the first 6 h, 33.3% within 12 h, 50% within 24 h, and 68.2% within the first 2 days after admission. Six patients were operated on, with a perioperative mortality of two of six patients and a 5-year survival of two of six patients. All patients who were not operated on died. Pain was the most frequent initial symptom. Every patient had some kind of cardiovascular and respiratory sign. Neurologic symptoms occurred in 28 of 66 patients (42%). Five patients presented with clinical pictures of acute abdomen and two with acute renal failure. Trunk arteries were affected in 33 of the 80 autopsied cases (41%), and rupture of aorta was seen in 69 cases (86%). In five cases, spontaneous healing of dissection was also found. The ratio of proximal/distal dissections was 5.1 to 1. All 18 prehospital cases were acute. Fifty-nine cases (89.4%) were acute at admission, and 7 cases (10.6%) were chronic dissections. Hypertension and advanced age were the major predisposing factors. Aortic dissection was the initial clinical impression in only 13 of the 84 patients (15%). Thus, 85% of the patients did not receive immediate appropriate medical treatment. For this reason, these late-recognized and/or unrecognized cases may be regarded as an untreated or symptomatically treated group, whose course may resemble the natural course of aortic dissection.