The resting energy expenditure (REE) of 43 men and 41 nonpregnant women aged 18-42, of the Iban tribe in rural Sarawak, was measured using the Oxylog and analyzed in relation to fat mass (FM) and fat-free mass (FFM) derived from measurements of skinfold thickness, to estimates of the specific energy expenditure of the FFM, and to microclimatic variables. Measured REEs exceeded values predicted from FFM by 9-22%, but were consistent with the metabolic rates expected of Western adolescents of comparable body size and composition. In both sexes, body fat content significantly influenced REE after accounting for nonlinear effects of variation in FFM. This is consistent with reported high adipocyte metabolic rates in obese Westerners, and with the hypothesis that Iban may possess a relatively large capacity for fat mobilization and storage. Therefore, in some populations, terms representing both FM and FFM should be included in equations describing REE. These findings are consistent with postulated evolutionary advantages of efficiency in regulation of energy turnover. © 1993 Wiley-Liss, Inc.