There are remarkable world-wide differences in the prevalence of diabetes, ranging from virtually 0% in Papua, New Guinea, to over 50% in the Pimas of Arizona. There are also notable rural-urban (e.g. Polynesians in rural and urban Western Samoa) and native-migrant (eg Chinese in China and in Mauritius) differences. The reporting of a high prevalence of diabetes in many populations that have undergone either urbanization or migration suggests that environmental factors related to lifestyle are contributory. Two such factors may be physical inactivity and dietary animal fat. There are also remarkable differences in ethnic susceptibility to non-insulin dependent diabetes mellitus (NIDDM), indicative of a strong genetic factor. Obesity is a risk factor for NIDDM. In addition to body weight, however, the pattern of distribution of body fat must also be considered. In some ethnic groups a central pattern of body fat distribution has been shown to be a significant risk factor even in the absence of obesity. This is particularly true in many Asian populations. It is proposed that genetic predisposition plus environment (lifestyle) interact and lead to visceral adiposity and insulin resistance, and that heredity and lifestyle also interact to cause a beta-cell lesion that affects insulin production and secretion. Development of insulin resistance facilitates the emergence of the beta-cell lesion. A consequence of this sequence of events is the development of glucose intolerance, and eventually, NIDDM.