The epidemic of type 2 diabetes among children, adolescents and adults is increasing along with the increasing prevalence of overweight and obesity. Overweight is the most powerful modifiable risk factor for type 2 diabetes. Intake of whole-grain foods may reduce diabetes risk. Three prospective studies in 160 000 men and women examined the relationship of whole-grain or cereal-fibre intake with the risk of type 2 diabetes. Each study used a mailed Willett food-frequency questionnaire and similar methods of quantifying whole-grain foods and cereal fibre. The self-reported incident diabetes outcome was more reliably determined in the two studies of health-care professionals than in the study of Iowa women. Risk for incident type 2 diabetes was 21–27% lower for those in the highest quintile of whole-grain intake, and 30–36% lower in the highest quintile of cereal-fibre intake, each compared with the lowest quintile. Risk reduction persisted after adjustment for the healthier lifestyle found among habitual whole-grain consumers. Observations in non-diabetic individuals support an inverse relationship between whole-grain consumption and fasting insulin levels. In feeding studies in non-diabetic individuals insulin resistance was reduced using whole grains or diets rich in whole grains. Glucose control improved with diets rich in whole grains in feeding studies of subjects with type 2 diabetes. There is accumulating evidence to support the hypothesis that whole-grain consumption is associated with a reduced risk of incident type 2 diabetes; it may also improve glucose control in diabetic individuals.