Positron emission tomographic studies using metabolic ratios to represent the contrast between certain brain regions have shown differences of cerebral glucose metabolism between presenile and senile dementia of Alzheimer type (DAT). In this study, factors for regional variance of cerebral metabolism in 61 patients with probable Alzheimer's disease were obtained using principal component analysis (PCA). The difference between two age groups of DAT patients according to age at onset less than or equal to 65 years (n = 30) and > 65 years (n = 31) were investigated. A PCA using nine pairs of cerebral regions normalized to cerebellum for reduction of intersubject variability identified five regional factors for cerebral metabolism: one each for the left and right temporo-parietal cortex, and the other three for frontal cortex, primary visual and sensorimotor cortex, and mesial temporal cortex. The age groups differed significantly only in the factor for the right temporo-parietal cortex, with a more prominent metabolic reduction in presenile than senile DAT. Severity of dementia was correlated with the factors for the right and left temporo-parietal cortex. The effect of age at onset on glucose metabolism of the right temporo-parietal cortex was independent from that of dementia severity in DAT.