An altered equilibrium of protease/protease-inhibitor factors may be involved in the pathogenesis of aneurysm rupture: alpha 1-antitrypsin (alpha 1-AT) represents the most relevant inhibitor of elastase, a proteolytic enzyme enhancing catabolic processes of collagen metabolism. In the present study we test the hypothesis whether the activity of alpha 1-AT is altered in SAH patients; 5 cases with unruptured intracranial aneurysm and 27 patients with diagnosis of aneurysm SAH were included in the study. Blood samples were obtained immediately at admission. As control samples we consider the 5 cases of unruptured aneurysm, 15 cases of unruptured aortic aneurysms and 10 patients with non-vascular CNS diseases. Measurement of alpha 1-AT level was determined by immunoturbidimetric method. Serum levels of alpha 1-AT are significantly lower in patients admitted within 72 hours after SAH, if compared to patients admitted in a delayed phase. The linear relationship between alpha 1-AT and collagenase inhibitory percentage capacity (CIC) was shown to be different in the 4 subgroups considered, and so were the mean % CIC values in the between-groups comparison, except for unruptured aneurysm vs controls. The alpha 1-AT CIC in patients with SAH is shown to be the lowest when compared to controls and unruptured aneurysms (p = 0.0001).