The risks of microvascular complications over periods of 3 and 6 years in relation to baseline plasma glucose concentrations were estimated in Pima Indians of Arizona aged 25 years or more and who were not taking hypoglycaemic medicine at their initial examinations. Retinopathy (microaneurysms, haemorrhages, or neovascularisation) developed within 3 years in 1% of 181 subjects with initial fasting plasma glucose concentrations less than 140 mg/dl (1 mg/dl = 0.0555 mmol/l) and in 13% of 39 subjects with higher initial fasting plasma glucose levels. No one with an initial 2 h glucose concentration less than 200 mg/dl had retinopathy within 3 years, but the rate was 12% in 60 subjects with higher glucose concentrations. Retinopathy developed within 6 years in 1% of 446 subjects whose 2 h post-load glucose concentrations were less than 200 mg/dl and in 20% of 113 subjects with higher levels. Subsequent development of heavy proteinuria (urine protein/creatinine ratio at least 1.0) was less common than that of retinopathy, but was similarly related to the initial fasting and 2 h plasma glucose concentrations. The finding that these microvascular complications rarely appeared in individuals with fasting plasma glucose concentrations less than 140 mg/dl or 2 h plasma glucose concentrations less than 200 mg/dl supports the validity of these concentrations as part of the new diagnostic criteria for diabetes mellitus.