The pathogenic basis of the association between adult polycystic kidney disease (APKD) and cerebral aneurysms is unknown. We have compared cerebral aneurysms in 79 patients with APKD gleaned from the literature to the sporadic aneurysm cases reported by the Cooperative Study to determine if there are significant biological differences between these two groups. Sixty-eight patients had a single aneurysm and 11 (14%) had multiple aneurysms. In APKD patients with subarachnoid hemorrhage from a single aneurysm there was a significant over-representation of males (72%, p less than 0.01); and the APKD group had more aneurysms of the middle cerebral artery (37%, p less than 0.05). The peak decennial incidence and mean age of rupture of APKD-associated aneurysms was younger (mean age 39.7 years, p less than 0.01) and over 77% of APKD-associated aneurysms had ruptured by age 50 versus 42% for sporadic aneurysms (p less than 0.001). Cerebral aneurysms co-existed with APKD in the absence of hypertension in 25% of 45 cases where the presence or absence of hypertension was recorded. These biological differences and the occurrence of aneurysms in normotensive APKD patients suggests an etiology which may be independent of hypertension and that APKD-associated aneurysms may be genetically determined. It is hypothesized that cases of inherited, familial cerebral aneurysms could be linked to a genetic defect resembling that which occurs on chromosome 16 in APKD.