In order to characterize spontaneous adult-onset obesity in a non-human primate model, we have studied a group of twenty-four obese and non-obese male rhesus monkeys (Macaca mulatta). The monkeys, ranging in age from 12 to 27 years, were defined as obese on the basis of tritiated water estimates of body fat content exceeding 25 percent of body weight. Although the obese and non-obese monkeys had similar crown-rump lengths, they differed significantly not only in body weight (17.0 +/- 3.2 vs 11.7 +/- 1.8 kg, X +/- s.d., P less than 0.001), and average body fat content (37.8 +/- 6.6 vs 13.2 +/- 5.4 percent, P less than 0.001) but also in midgirth circumferences (57.5 +/- 8.4 vs 34.8 +/- 6.2 cm, P less than 0.001) and abdominal (but not triceps or scapular) skinfold thicknesses (22.74 +/- 5.8 vs 9.82 +/- 1.82 mm, P less than 0.001), thus, indicating the predominantly abdominal distribution of the fat mass. A new Obesity Index Rh, for rhesus monkeys, defined as body weight divided by the square of the crown-rump length, was developed as an adaptation of obesity indices used for humans and rats. The high correlation of the Obesity Index Rh with percent body fat and its relative independence of height make possible future identification of obese rhesus monkeys on the basis of anthropometric measurements. There were slight, but not significant, differences between the obese and the non-obese groups in lean body mass (10.9 +/- 2.8 vs 8.8 +/- 1.8 kg) and in fasting plasma glucose levels (87.1 +/- 31.8 vs 63.2 +/- 7.5 mg/dl). Obese monkeys had significantly larger average fat cell sizes (1.29 +/- 0.54 vs 0.61 +/- 0.29 microgram lipid/cell, P less than 0.05) and significantly greater fat cell numbers (6.1 X 10(9) vs 2.2 X 10(9), P less than 0.01). Fat cell numbers were better correlated with body weight and total body fat parameters than fat cell size, while fat cell size was more closely associated with the log of fasting plasma insulin levels than was fat cell number. The similarities to studies in humans indicate the importance of the spontaneously obese adult rhesus monkey as an animal model in the study of obesity.