Recent epidemiologic studies have revealed a high prevalence of maturity-onset diabetes in certain populations that have undergone comparatively rapid urbanization. There is evidence suggesting that Australian Aborigines may respond to urbanization in this way. Thirteen full-blood Aborigines from the Mowanjum Community, Derby, Western Australia, cooperated in the present study. They spent 3 mo living in their traditional hunter-gatherer life-style, after which their insulin response to glucose was measured in a starch tolerance test. The findings were compared in follow-up studies conducted 3 mo after returning to their urban environment. Similar studies were conducted in Caucasians of comparable age and weight. Fasting glucose concentrations were lower in Aborigines than in Caucasians and were unaffected by life-style changes. Although basal insulin levels were similar in the three groups, there were striking intergroup differences in the insulin responses to glucose. The areas under the insulin curves in the first hour after starch ingestion were: urban Aborigines 4478 +/- 465 microU/ml-1/min, traditional Aborigines 2959 +/- 301 microU/ml-1/min, and Caucasians 2097 +/- 224 microU/ml-1/min. This appeared to reflect differences in the early rates of change of glucose concentrations. The data suggest that these Aborigines have an abnormally high insulin response to glucose, which is ameliorated, but not normalized, by reverting to their traditional life-style.