It is an autosomal recessive genetic disorder characterized by a deficiency in the enzyme hepatic phenylalanine hydroxylase (PAH), necessary to metabolize the amino acid phenylalanine. The phenylalanine excess and tyrosine deficiency can cause problems with brain development, leading to progressive mental retardation, muscle hypothomy and seizures. A "musty or mousy" odor and a tendency to hypopigmentation and eczema are also observed. Such symptoms are present only with untreated PKU. The Guthrie test, widely applied on the third day after the baby’s birth, either excludes or confirms the presence of phenylketonuria. A diet low in phenylalanine and high in tyrosine is introduced within 2 weeks from the birth. The diet causes the elimination of harmful influence of phenylalanine on the brain, and most physicians now agree that this special diet should be followed throughout life.

The organism of the person affected with phenylketonuria is not able to metabolize one of diet’s elements – phenylalanine. This requires severely restricting or eliminating foods high in phenylalanine, such as meat, eggs, fish, milk, cheese, and (in smaller amounts) other cereal products, fruit and vegetables.

Excessive concentration of both phenylalanine and the products of its decomposition in blood is a cause of physical and mental development disorders.