Perhaps no inherited disorder has been evaluated more thoroughly, nor demonstrated more decidedly gene-enzyme specificity dependent on specific environmental conditions and stimulus, than phenylketonuria. Yet many questions remain unanswered, one of the important of these, being specific behavioral aspects of the disease, if any, and their biopsychogenesis. Discovered in 1934 by Dr. Asbjörn Fölling of Oslo, with the help of a perspicacious Norwegian mother who noted a peculiar odor from urine of two of her offspring, phenylketonuria has now become a preventable form of mental retardation.8Approximately 700 cases have been mentioned in the literature which abounds with articles concerning various aspects of the disease. The abnormality, the result of homozygous recessive autosomal inheritance6,21occurs once per 25,000 to 40,000 births in the United States,21or in a ratio of 2 to 6 per 100,000 population,21,37accounting for 0.6%