A diet rich in fat may be an important precipitating factor of obesity, but studies on this relation have been hampered by the lack of an objective method to assess habitual dietary fat content. We measured 24-h fat oxidation in a respiration chamber in 38 overwight or obese and 35 nonobese women, and used it as an estimate of habitual dietary fat energy (%). After adjustment for confounders, obese women had higher oxidative fat energy than nonobese women [40.2% (37.8-42.6) vs. 36.0% (33.6-38.5), p < 0.02]. Adjusted oxidative fat energy (%) increased with increasing size of fat mass, and this relation suggest that a 10-kg change in fat mass may be caused by a change in dietary fat energy of > or = 1.6%. This objective assessment supports the contention that obese subjects consume a diet with a higher fat content than nonobese individuals, and the high-fat diet may have causal importance for the development and maintenance of obesity.