Aortic elastase, antiprotease (a-1-antitrypsin) and total protein were assayed in 37 patients who underwent an operation on the abdominal aorta for ruptured abdominal aortic aneurysms (AAA), an elective procedure for AAA or aortofemoral bypass for occlusive disease. Aortic elastase modified by local antiprotease activity (elastase/a-1-antitrypsin) was significantly higher in patients with a ruptured AAA compared with patients with an elective AAA or occlusive aortic disease: 241 nanograms per milligram of tissue in AAA-rupture versus 57 nanograms per milligram of tissue in AAA-elective versus 32 nanograms per milligram of tissue for occlusive (p less than 0.003). Patients with a ruptured AAA had the highest elastase activity (354 nanograms per milligram of tissue) and the lowest a-1-antitrypsin (1.07 nanograms per milligram of tissue) compared with patients with an elective AAA and occlusion (p less than 0.05). These data suggest that the homeostatic balance between elastase and antiprotease may be significantly altered in the aortic wall at the time of aneurysm rupture. Increased elastase unchecked by low antiprotease results in increased elastin breakdown which may be the inciting event for rupture of a compromised, thin aortic aneurysm wall.