However, as PKU is left untreated, the abnormal buildup of phenylalanine can cause perminant mental retardation and other neurological problems, such as seizures, in infants. As these babies grow and develop, other signs of the disease are evident. These include a clumsy manner of walking, vomitting and a musky body odor which is due to phenylalaninic acid in sweat and urine.

Additionally, patients with PKU tend to be hyperactive and some children are diagnosed with ADD due to the high levels of phenylalanine in the bloodstream. Also, high levels can decrease the production of a neurotransmitter responsible for mood maintenance, which can lead to depression. It is also very common for PKU patients to develop eczema, a skin disorder that causes rashes and bisters.

All symptoms develop gradually, so if not detected at infancy, it is often difficult to realize that a person has PKU until irreversable mental retardation has occured.