Regional cortical function was evaluated in 13 patients with primary degenerative dementia and six age-matched, neurologically normal control individuals. Five patients were selected on the basis of preponderant signs of posterior dementia, five had mainly signs of anterior dementia, and the remaining three evidenced mixed anterior-posterior cognitive deficits. Positron emission tomographic scans following the intravenous administration of fluorodeoxyglucose revealed a characteristic pattern of abnormalities in each group: those with posterior dementia, as primarily manifested by verbal and visuospatial deficits, had a predominant focus of hypofunction in the posterior parietal and temporal regions; those with the less common anterior dementia, evidenced by prominent personality and social behavioral changes, had their most conspicuous abnormality in the anterior frontal cortex; those with approximately equal signs of anterior and posterior dementia had abnormalities in both anterior frontal and posterior parietal-temporal areas. The results indicate that the primary degenerative dementias may reflect a selective abnormality of the frontal and parietal association corticies. The continuous distribution of cortical involvement in patients with these disorders suggests that diagnostic entities as neuropathologically distinct as Alzheimer's disease (mainly posterior involvement) and Pick's disease (mainly anterior involvement) could share a common pathophysiology. A focusing of histological and biochemical efforts in areas of major functional abnormality could hasten the elucidation of such mechanisms and the development of effective therapies.