Members of the Pima Indian population are obese, on average, as estimated by the body mass index (BMI). Young adults have had the highest BMIs and there have been modest increases in age- and sex-specific mean BMIs for the past 25 y. These observations suggest that the older adults have had less exposure to factors leading to obesity than have the younger adults. Compared with children studied early in this century, present-day Pima children are much heavier for height, suggesting that the degree of obesity has increased since that time. Obesity in the Pimas is familial and has complex relationships with non-insulin-dependent diabetes mellitus, a common disease in this population. Obesity predicts the development of diabetes; once people have diabetes, however, they tend to lose weight. Thus, obesity should not be studied in this population without also considering diabetes, which tends to limit the degree of obesity.