Glucose metabolism was examined by positron emission tomographic scanning with F-2-fluoro-2-deoxy-D-glucose in 29 persons at risk for Huntington's disease (HD), 28 age-matched controls, nine patients with stage I, and eight patients with stage II symptomatic HD. Absolute caudate metabolic rates and normalized indexes of caudate metabolism for at-risk persons were normal compared with controls. No at-risk person had caudate indexes outside two SDs of the controls' mean. Caudate metabolism in the earliest HD cases was significantly reduced compared with controls and at-risk persons, but within the 99% confidence levels of both groups. Stage II patients had caudate measures that were significantly depressed compared with those of stage I HD patients. Measurement of caudate glucose hypometabolism is unlikely to be sufficiently sensitive to serve as a presymptomatic marker of heterozygote status, although it will provide a sensitive marker for progressive caudate dysfunction in HD.