A deficiency of alpha 1-antitrypsin has been implicated in the development of arterial aneurysms, including intracranial aneurysms. The authors determined the prevalence of alpha 1-antitrypsin deficiency of different phenotypes in 100 consecutive patients with intracranial aneurysms and compared the distribution of alpha 1-antitrypsin phenotypes to that in the general population (904 people). The study population consisted of 44 men and 56 women with a mean age of 52 years (range 15-81 years). The heterozygous alpha 1-antitrypsin deficiency states (PiMS and PiMZ) were more common in patients (16%) than in the general population (7%), providing an odds ratio of 2.56 (95% confidence interval (CI) 1.32-4.75; p = 0.005). In addition, one patient (1%) was homozygous for the deficient allele (PiZZ) compared to an expected number of 0.015, providing an odds ratio of 67.0 (95% CI 2.0-363.3; p = 0.015). These findings lead the authors to suggest that the heterozygous and homozygous alpha 1-antitrypsin deficiency states are genetic risk factors for the development of intracranial aneurysms.