The Hoorn study has investigated the prevalence of impaired glucose tolerance (IGT) and diabetes in a random sample of the population, 50-75 years of age, in the small Dutch town of Hoorn. The percentage of the population identified as having IGT was 10.3%, while 8.4% had diabetes. The prevalence of both diabetes and IGT was higher in older subjects (70-74 years of age) than in younger subjects (50-54 years of age), suggesting that age is a major determinant of the prevalence of IGT and diabetes in a population. A prospective study of the development of non-insulin dependent diabetes mellitus (NIDDM) in the subjects identified as having IGT was then initiated. Preliminary results for the 158 subjects who have been followed for a mean of 2 years indicated that 28.5% (95% confidence interval, 15-42%) have progressed to NIDDM within this period. The progression rate calculated from this value is 13.8%/year (95% confidence interval, 3.5-24%/year). Analysis of possible determinants of conversion has revealed that 2-hour plasma glucose levels greater than 9.4 mmol/litre and increased fasting proinsulin levels are predictive of progression to NIDDM. This suggests that beta cell dysfunction, rather than increased insulin resistance, is responsible for the development of NIDDM.