Young normal, elderly, and clinically diagnosed Alzheimer disease subjects who had undergone positron emission tomography (PET) and computed tomography (CT) examinations were studied to determine the effect of periventricular white matter lesions on cerebellar glucose metabolic rates. PET-determined cerebellar metabolic rates were elevated in subjects with periventricular white matter lesions. These results suggest the cautious use of cortical-to-cerebellar ratios in future PET or single-photon-emission CT (SPECT) studies.