Phenylpyruvic oligophrenia1is a rare, inherited, metabolic fault that appears first in childhood. It involves inability to convert properly the essential amino acid, phenylalanine, into tyrosine and is associated with excretion of phenylpyruvic acid and other abnormal metabolites in the urine. Affected persons are invariably mentally deficient or idiotic2-6and are almost uniformly blond, light-skinned, and blue-eyed.3,5,7The etiologic relationship to mental deficiency is unexplained6; the relationship to blondness is thought to lie in the inability to convert phenylalanine properly into tyrosine in the process of forming melanin. Certain peculiarities of the skin also have been described in association with phenylketonuria, but this paper is chiefly concerned with the pigmentary aspects of the disorder. In 1934, Fölling,8of Norway, first described the presence of phenylpyruvic acid in the urine of certain mentally defective patients. A characteristic deep green color appeared