ABSTRACT
Phenylketonuria is an inborn error of phenylalanine catabolism causing irreversible mental retardation which can be prevented by dietary therapy. It is characterized by excretion of phenylpyruvic acid in the urine and high concentration of phenylalanine in the blood. Phenylketonuria contributes significantly to the mentally-retarded population. The urine of four [4] of the thirty-two [32] mentally retarded children in a mentally-retarded children home ["Terry Home"] in Kano-city tested positive to phenylpyruvic acid detection test i.e. ferric chloride test. However, high concentration of phenylalanine was detected in three of the four children. These three children, constituting 9.4% f the mentally retarded children, were concluded to suffer from phenylketonuria