Recommended criteria for the diagnosis of maturity-onset diabetes mellitus based on glucose tolerance tests vary considerably; none are derived from long-term observations of the further development of different degrees of glucose intolerance. Evidence from several epidemiological investigations suggests that the risk of specific diabetic complications becomes important only in people with capillary whole-blood sugar concentrations exceeding 200 mg/dl 2 hours after a 50 g oral glucose load, who have overnight fasting blood-sugar concentrations usually exceeding 110 mg/dl. Lesser degrees of glucose intolerance may, nevertheless, indicate an additional risk of atherosclerotic arterial disease. Assessment of "borderline diabetics" should, therefore, include and evaluation of other known risk factors for arterial disease and any treatment programme should be determined in the light of these as well as by the degree of glycaemia.