Abstract
The excretion of phenylpyruvic acid by patients with phenylketonuria was roughly proportional to their fasting serum phenylalanine levels. Phenylpyruvic acid was not detectable in the urine of patients with serum levels of phenylalanine below 15 mg/100 ml. Children less than 3 years old frequently show much higher serum levels of phenylalanine than older patients. The incidence of known phenylketonuria in Utah during the past 10 years has been 1 in 20,400 births.