Venous plasma glucose concentrations before and two hours after a 75 g carbohydrate load were determined on 597 Nauruans, a Micronesian population with one of the highest diabetes prevalence rates yet recorded in the literature. A marked increase in both the mean fasting and postload plasma glucose concentrations with age for both sexes was noted for the total population. In this population the frequency distributions of fasting and two-hour plasma glucose show bimodality, separating the population into normal and hyperglycemic subpopulations. Analysis of mean fasting and two-hour glucose concentrations of subjects in the normal and hyperglycemic components shows that the increase in mean glucose with age in the Nauruan population is mainly the result of an increasing proportion of subjects falling into the hyperglycemic (diabetic) component with increasing age. Only small changes in mean glucose concentrations with age were noted in subjects in the normal component. Percentile distributions of both fasting and postload glucose concentrations indicate that there is a dramatic age-related increase in glucose concentrations in the higher percentiles (70th and 90th) in both males and females. This finding is consistent with the hypothesis of two subpopulations in which the glucose concentrations increase significantly with age in one (higher percentiles) and remain relatively constant in the other (lower percentiles). The data indicate that, while there is a rise in glucose concentrations with increasing age, it does not necessarily represent diabetes. The major contribution to this age-related rise is due to the increasing number of diabetics with age, and the diabetics can be separated from the rest of the population on the basis of percentile analysis and bimodality of glucose distributions.