To show a relationship between age and cerebral blood flow as well as between age and cerebral vasocapacitance to induced hypercapnia or cerebral reserve percentage, positron emission tomography (PET) with fluorine-18-fluoromethane was used to study 36 normal subjects and 36 patients, all free of stroke, who presented with transient ischemic attacks in a unilateral carotid distribution. Symptomatic-side data have been published elsewhere. All asymptomatic carotid arteries were either normal or had nonstenotic plaque, and subjects were free of intracranial stenoses. Cerebral blood flow and cerebral reserve percentage were calculated for both middle cerebral artery flow territories. Serum hematocrit, expired carbon dioxide tension, and mean arterial blood pressure did not distinguish the two groups of patients or show an age effect. For the 72 normal nonstenotic sides in the volunteers, linear regression showed that while increasing age was significantly related to decreasing cerebral blood flow in the middle cerebral artery, there was no particular relationship between age and decreasing cerebral reserve percentage. In patients, while no overwhelming linear relationships were found between age and decreases in either cerebral blood flow or cerebral reserve percentage, the latter values approached significance as declining cerebral reserve percentage did relate to increasing age. Over a 60-year range in normal subjects, cerebral blood flow declined 37% with age; there was a 21% age-related decrease when normal subjects 50 years and younger were compared to those 50 years and older.(ABSTRACT TRUNCATED AT 250 WORDS)