Because positron emission tomography (PET) provides measurements per unit volume of intracranial contents, these measurements may be affected by the inclusion of metabolically inactive CSF spaces in the volume in which they are made. Thus, PET measurements of CBF and metabolism may be artifactually lowered in normal aging and dementia, which are both associated with significant brain atrophy. We describe a method to correct global PET data, averaged over several tomographic slices, for cerebral atrophy by using measurements of CSF space volume obtained with quantitative x-ray computed tomography. The importance of making such a correction is demonstrated using PET measurements of CBF and oxygen metabolism obtained in normal young, normal elderly, and demented subjects.