This study investigated differences in resting metabolic rate (RMR) between obese nonobese females and between females with adult and childhood onset of obesity. Subjects were 18 healthy, Caucasian women, ages 20-38 (M=28.16), 6 from each of 3 groups: nonobese with no history of obesity (M-18.53% fat), child-onset obese (COO) (M=41.00% fat), and adult-onset obese (AOO) (M=37.8% fat). Subjects were nonsmokers, weight-stable for at least 3 months prior to the study, and not following a low-calorie diet. All obese subjects had lower-body obesity (waist/hip girth ratio < .80). Subjects underwent three measures: RMR by indirect calorimetry, residual lung volume, and hydrostatic weighing. In addition, a questionnaire elicited information about weight history, educational level, occupation, frequency of exercise, and activity level. When data from the 12 obese subjects were combined, RMR per kg body weight (RMR/BW) was significantly lower in the obese (18.47 kcal/kg/day) than the nonobese (22.94 kcal/kg/day). The obese subjects also had significantly lower RMR per kg fat mass (RMR/FM) than the nonobese. When data from COO and AOO subjects were analyzed separately, COO were found to have significantly lower RMR/FM than the nonobese. COO and AOO subjects did not differ statistically in absolute RMR, RMR/BW, RMR/FM, or RMR/FFM, although COO had lower values on all measures. Multiple regression analysis indicated that, in all groups, more of the variance in RMR was explained when the variables, FM and FFM. However, the effect of activity and exercise on explained variance in RMR was much smaller in COO women.