Fifteen drug-free patients with early to midstage Huntington's disease were evaluated with quantitative neurological examinations, scales for functional capacity, computed tomographic (CT) scans, and positron emission tomographic (PET) scans of 18F-2-fluoro-2-deoxyglucose (18F-FDG) uptake. All patients had abnormal indices of caudate metabolism on PET scanning, whereas in patients with early disease indices of putamen metabolism and CT measures of caudate atrophy were normal. Indices of caudate metabolism correlated highly with the patients' overall functional capacity (r = 0.906; p < 0.001) and bradykinesia/rigidity (r = −0.692; p < 0.01). Indices of putamen metabolism correlated highly with motor functions: Chorea (r = −0.841; p < 0.01), oculomotor abnormalities (r = −0.849; p < 0.01), and fine motor coordination (r = −0.866; p < 0.01). Indices of thalamic metabolism correlated positively with dystonia (r = 0.559; p < 0.05). The data suggest that PET scanning with 18F-FDG is a sensitive measure of brain dysfunction in Huntington's disease and that basal ganglia metabolism is highly correlated with the overall functional capacity of individual patients and with the degree of their motor abnormalities.