Risk factors for non-insulin-dependent diabetes mellitus (NIDDM) were assessed in a population of 5042 middle-aged white men, initially nondiabetic, who were followed 3 yr. The subjects were participants in the Paris Prospective Study I. Sixty-three subjects developed diabetes during the follow-up. Plasma glucose concentration in the years before the occurrence of the disease was a major risk factor. Subjects with normal glucose tolerance but elevated fasting plasma glucose exhibited a similar risk of developing NIDDM as did subjects classified as having impaired glucose tolerance on the basis of 2-h postload glucose. In a multiple logistic regression, a high fasting plasma insulin concentration and a low 2-h plasma insulin concentration after a glucose load in association with a high body mass index were independent predictors of conversion to NIDDM from impaired glucose tolerance. Previously, this result had been found only in Nauruans, Pima Indians, and Japanese. This demonstrates for the first time in a white population that a high fasting and low 2-h insulin concentration is predictive of conversion to NIDDM from impaired glucose tolerance.