Trends in the prevalence and incidence of non-insulin-dependent diabetes mellitus and impaired glucose tolerance were studied in Micronesian Nauruans aged 20 years and above, by linking glucose tolerance data collected during population surveys performed in 1975/1976, 1982, and 1987. Based on World Health Organization criteria, the age-standardized prevalence of non-insulin-dependent diabetes mellitus has remained relatively constant (24.0% in 1987), but the prevalence of impaired glucose tolerance has decreased significantly from 21.1% (95% confidence interval (CI) 17.0-25.3) in 1975/1976 to 8.7% (95% CI 7.1-10.3) in 1987. Between the periods 1975/1976-1982 and 1982-1987, the incidence of progression from normal glucose tolerance to either impaired glucose tolerance (incidence rate ratio (IRR) = 0.55, p less than 0.01) or non-insulin-dependent diabetes (IRR = 0.46, p less than 0.05) has decreased dramatically, while progression from impaired glucose tolerance to non-insulin-dependent diabetes has increased (IRR = 1.23). The overall age-standardized incidence of non-insulin-dependent diabetes has declined from 26.2 cases/1,000 person-years in 1975/1976-1982 to 22.5 cases/1,000 person-years in 1982-1987. As there were no changes in the frequency of recognized risk factors, the decline in incidence of glucose intolerance is probably due to the intensity of the epidemic in Nauru, which has already removed a high proportion of the genetically susceptible individuals from the pool with normal glucose tolerance. Coupled with the observations that mortality is higher and fertility lower in diabetic Nauruans across the age range, the observed decline in the incidence and prevalence of non-insulin-dependent diabetes mellitus and impaired glucose tolerance may presage a fall in the population frequency of the diabetic genotype, at least in its more severe form, as might be predicted on the basis of Neel's "thrifty genotype" hypothesis.