alpha 1-Antitrypsin (alpha 1-AT) deficiency may play a role in arterial aneurysmal disease by allowing increased proteolysis of arterial structural proteins. Alpha 1-AT levels are influenced by variation at the PI (protease inhibitor) locus. PI phenotypes were determined in 173 patients with abdominal aortic aneurysms (77 from Pittsburgh, 96 from London) and in 72 patients with intracranial aneurysms (26 from Pittsburgh, 46 from London). No excess of PI deficiency alleles was observed in either of the aortic aneurysm data sets or in the Pittsburgh intracranial aneurysm data. The PI*Z deficiency allele frequency in the London intracranial aneurysm data was 8-fold higher than in controls; however, this was not significant after correcting for multiple comparisons. PI phenotype had no effect on aneurysm age-at-diagnosis within any of the data sets. Smoking history had an effect on aneurysm age-at-diagnosis only within the Pittsburgh intracranial-aneurysm data.