The effects of a 20%-fat diet (LF) on total body weight, lean body weight, and adiposity were studied in 18 premenopausal women with body mass index (BMI) of 18-44. Subjects were fed a 37%-fat (HF) control diet for 4 wk followed by the LF diet for 20 wk. Total body weight, lean body weight, and fat weight were measured at the end of the HF and LF dietary periods by hydrostatic weighing. Despite adjustments in energy intake to maintain weight throughout the study, subjects exhibited a 2.8% decrease in total body weight (P less than 0.0006), an 11.3% decrease in fat weight (P less than 0.0001), and a 2.2% increase in lean body weight (P less than 0.0149) by the end of the LF period. Similar changes were observed in obese (BMI greater than 30) and non-obese women (BMI less than 30). By the end of the LF period, energy intake had increased significantly in comparison with the HF diet (119% of the HF intake, P less than 0.0001). Results could not be explained by changes in daily activity levels and suggest that macronutrient composition plays a role in energy requirements for weight maintenance.