Amyotrophic lateral sclerosis and Parkinsonism-dementia are unusually prevalent on Guam. Carbohydrate metabolism was studied in 110 patients with evidence diagnostic of or suspecious for these diseases. The combined incidence of known diabetes in 29 per cent of them plus a high percentage of glucose tolerance tests interpreted as abnormal, even when most age-related criteria were considered, was considerably higher than the incidence of abnormal carbohydrate metabolism reported elsewhere in the general population of the United States, the tropical Pacific area, or in recent surveys on Guam itself. The diabetes was generally mild in nature and noteworthy for a lack of retinopathy and other complications. Hypertension, hypercholesterolemia, and hyperuricemia, although highly prevalent, were not consistently associated with abnormal glucose metabolism. Similarly, no consistent association was demonstrated with such factors as age, muscle atrophy, or physical activity.