The effect of age on regional cerebral metabolic rate of glucose (rCMRGl) was studied in 14 patients with presenile dementia of Alzheimer type (DAT) and 24 patients suffering from senile DAT in comparison to 20 age-matched normal subjects by positron emission tomography (PET) of 2-(18F)-fluoro-2-deoxy-D-glucose (FDG). The metabolic pattern was condensed to a single metabolic ratio. It was calculated as the quotient of rCMRGl in regions typically affected by AD (frontal and temporoparietal cortex) divided by that in regions typically not affected. In normals this ratio was 1.05 +/- 0.04 and did not depend on age. In patients, the metabolic ratio was generally smaller and there was a significant difference between presenile (0.82 +/- 0.1) and senile DAT (0.90 +/- 0.1). This was due to a different metabolic pattern in the two age groups: metabolic impairment was focused on frontal and temporo-parietal cortex in presenile DAT, whereas more global rCMRGl reductions were present in senile DAT. The results suggest a more generalized disorder in senile dementia impairing metabolism globally in addition to the more localized changes that are typical for DAT.