Two groups of children with phenylketonuria were followed from birth for several years. The Recommended Dietary Allowance group received a protein intake as recommended by the Food and Nutrition Board. The Food and Agricultural Organization (FAO) group received a protein intake as recommended by FAO. The children were followed very closely for the biochemical control of the disease. The children were also followed very closely to evaluate the adequacy of the protein intake using length, weight, routine hematology, chemical analysis, and x-ray of the hand. The results indicated two groups of healthy children. However, a decline in length growth percentile was found in some of the FAO children. A possible osteoporosis developed in two of the FAO children. The possible conclusion that the FAO "safe level of intakes of egg or milk protein" is marginal is discussed.