Regional cerebral blood flow (rCBF) was studied in 60 elderly persons (aged 65 to 84 years) recruited from a population-based study, with single photon emission computed tomography using technetium 99m-labeled hexamethylpropylene amine oxime. We investigated whether it is only age that affects rCBF or whether other factors can be indentified that explain this relationship. Using multiple linear regression analysis, increasing age was significantly associated with rCBF decrease in parietal, temporo-parietal, and temporal cortex, but not in frontal cortex. Adjustment with several risk factors for cerebrovascular disease, including hypertension, history of myocardial infarction, factor VIIc, factor VIIIc, cholesterol and HDL cholesterol, smoking, and diabetes mellitus had no influence on these relations. Conversely, the association between age and rCBF was no longer statistically significant after adjustment with fibrinogen and indicators of carotid atherosclerosis, including intima-media wall thickness of the carotid artery and plaques in the carotid artery. Correction with local ratings of cortical atrophy did not affect the relations between age and rCBF. The results suggest that in the elderly population rCBF declines with age in posterior cortical areas and that these changes may well be explained by the presence of atherosclerosis. Reduced contractility of the vascular muscle wall with increasing age resulting from atherosclerosis may be the underlying mechanism.