1. Aborigines from all over Australia develop unusually high prevalence rates for obesity, non-insulin dependent diabetes mellitus (NIDDM) and cardiovascular disease when they make the transition from their traditional hunter-gatherer lifestyle to a Westernized lifestyle. 2. There is no evidence that they experienced these diseases as hunter-gatherers. Data from the few extant traditionally orientated groups in very remote areas of northern Australia indicate that they were very lean (BMI less than 20 kg/m2), with low blood pressure, and that neither blood pressure nor BMI rose with age. 3. In addition, fasting glucose and cholesterol levels were low (usually below 4 mmol/L). However, in view of their extreme leanness and regular physical activity, fasting insulin and triglyceride levels were inappropriately elevated, suggestive of insulin resistance. 4. When Westernized diabetic Aborigines reverted temporarily to a traditional hunter-gatherer diet and lifestyle, all of the metabolic abnormalities of diabetes were greatly ameliorated (fasting glucose and triglyceride levels fell markedly and glucose tolerance and insulin secretion improved). In addition, they lost weight and there was a reduction in the major risk factors for cardiovascular disease (reduction in hypertriglyceridaemia and blood pressure, increase in bleeding time). 5. The hunter-gatherer lifestyle was associated with increased physical activity and a low-fat, high-fibre diet of low-energy density and high-nutrient density derived from very lean wild meat, and uncultivated vegetable foods. It has important therapeutic implications for the treatment and prevention of many of the chronic degenerative diseases of affluent Western societies.