PKU is a hereditary genetic disease.

A person with PKU has a genetic defect making it impossible for them to metabolize phenylalanine, an amino acid. The phenylalanine builds up in the person's body, especially in the blood plasma and the cerebrospinal fluid.

This overconcentration causes damage to the nervous system, causing mental retardation and seizures. Also, melanin production is lower, making children with PKU often blond, blue-eyed, and light-skinned.

The disease is transmitted by an autosomal recessive gene, meaning both parents must give the faulty gene to the child for them to get the disease. Treatment of the disease is a special diet low on phenylalanine; this rules out meat, dairy, and other foods high in protein. Protein drinks must be used to make up for the protein imbalance. This diet must be continued until adolescence.