We measured cortical glucose metabolism with positron emission tomography in 39 patients with Huntington's disease (HD) and in 34 controls. In the 23 patients with symptoms for less than 5 years, there was a 15% decrease in metabolism in frontal and inferior parietal cortex. In 16 patients with symptoms for more than 5 years, all cortical areas (except temporal) were significantly involved, with metabolic rates 25 to 30% below those of controls. These data indicate the presence of a diffuse abnormality of cortical function with early involvement of frontal lobes in HD, suggesting that the clinical manifestations may not be related solely to basal ganglia pathology, even in early disease.