Summary: The quantity and quality of protein in the diet affect both nitrogen balance and energy utilization in man. One possible mechanism is by a direct effect on the process of fat digestion and absorption. The effect on fat absorption of feeding diets providing 0%, 6.4%, or 12.5% of energy as protein was assessed in nine children. Liquid diets were prepared by high speed blending of calcium caseinate, a soy-cottonseed oil blend (80:20), sucrose, water, vitamins, and a mineral mixture and fed by bottle five times per day. Diets varied in the amount of protein and were fed to maintain constant the intakes of fat and carbohydrate. All children began by consuming the diet providing 6.4% of energy as protein for a period of 6 days: 3 days of adaptation, 3 days of metabolic collection of urine and stool. Successive 3-day dietary periods and metabolic collections followed in one of two sequences: 12.5%, 0%, 6.4% energy as protein in five children; 0%, 12.5%, 6.4% energy as protein in four. Mean fat excretion during the two 6.4% periods did not differ significantly (initial 14.5 ± 6% of intake; final, 11.7 ± 5.0% of intake). Fat excretion was significantly greater during the period when no protein was consumed (23.9 ± 7.7% of intake) than during either the period in which protein provided 6.4% of energy (P < 0.01) or during that in which protein provided 12.5% of energy (10.0 ± 2.6% of intake, P < 0.001). The difference between the 6.4% and 12.5% periods did not attain significance (P = 0.10). Fecal fat expressed as a percent-age of fecal dry weight was significantly greater during periods when the diet contained no protein than during periods in which protein provided either 6.4% (P < 0.05) or 12.5% (P < 0.01) of energy. Additional studies were carried out in a similar fashion in six children consuming diets in which protein provided 5%, 9.6%, and 15% of energy. All six possible dietary sequences were used. At these intakes fat excretion did not vary significantly with protein intake. Speculation: The low protein intake of children in much of the developing world may be detrimental to maximal fat absorption and consequently may increase their energy requirements. © 1977 International Pediatric Research Foundation, Inc.