Seven to eight thousand Pima Indians are presently living in the southwestern desert of Arizona. The prevalence of type II diabetes in this population exceeds 45% and more than 75% of the Pimas are obese. In 1962, Neel proposed that obesity in populations like the Pima Indians might be the expression of a "thrifty gene" which becomes detrimental with progress. Since 1982, longitudinal studies have been conducted including measurements of metabolic rate in 200 non-diabetic Pima Indians and have shown that 1) at any given body weight and body composition, there is quite a large variability in the resting metabolic rate which is not accounted for by intra-individual variability or errors of the methods; 2) metabolic rate after adjustment for body composition and body weight is a familial trait; 3) a low metabolic rate is a risk factor for body weight gain; 4) in response to body weight gain, there is a "normalization" of the resting metabolic rate. These studies are the first showing that a "thrifty" metabolic rate can play a role in the development of obesity.