Summary 1. A brief review of the literatureon phenylketonuria is presented with special emphasis on the spectrum of mental retardation. 2. An unusual case of phenylketonuriawas found to be of borderline intelligence after evaluation chemically, physically, and psychologically. 3. Serum phenylalanine levels didnot correlate with the patient's I.Q. 4. Psychological evaluation foundthe patient high in ability for rote memory and gives hope that increased mental age may be obtained with continued environmental security. 5. The patient was not treated withlow phenylalanine diet because of poor experimental results with use in older children, in addition to the fact that she is asymptomatic. 6. Phenylpyruvica oligophrenia isno longer correct terminology for the syndrome of phenylketonuria.