Two diets consisting of natural foods that differed in fiber content were fed to 12 men for 6 wk each in a crossover design. Diet 1 contained 37 g total dietary fiber (TDF) from fruits and vegetables and diet 2 contained 16 g TDF from fruit and vegetable juices in a 2800-kcal menu. Seven-day food, urine and fecal composites were made and analyzed for their gross energy, fat, protein and carbohydrate content during the fourth and fifth weeks of each dietary consumption period. The metabolizable energy (ME) of these diets was determined (gross energy of the diet - gross energy of urine and feces) and calculated by Atwater's general energy conversion factors (4, 9, 4), by specific energy conversion factors used in a U.S. food table and by formulas derived by various researchers. The coefficients of availability of energy and the energy-containing nutrients, fat, protein and carbohydrate, were lower while subjects were consuming the higher fiber diet. The specific energy conversion factors of the U.S. food table gave better estimates of the true ME of the lower fiber diet than the higher fiber diet, overestimating the ME of these diets by only 6% (diet 1) and 4.6% (diet 2).