The energy expenditure (EE) of eight lean, eight adult-onset obese (AOO), and eight child-onset obese (COO) women was determined over three 24-h periods by the factorial method, modified by subject-keypunched and mechanically recorded activity diaries. Mean daily EE was significantly higher in the pooled obese women (2472 +/- 488 kcal) than in lean women (1979 +/- 302 kcal) due to higher energy costs of sedentary and light activity in the obese. EE during moderate-to-strenuous activity was similar between groups because lean women performed these activities more vigorously. AOO and COO differed significantly in neither mean EE nor habitual activity. Fat-free mass (FFM) was a better predictor than body weight of both mean daily EE and the energy cost of activity. These data indicate that EE is positively related to obesity. Obese women tend to limit possible EE by reducing the vigorousness of weight-supported activity.