Phenylketonuria (PKU) or hyperphenylalaninemia (HPA) is a metabolic disorder caused by a deficiency in the phenylalanine hydroxylase (PAH) enzyme, resulting in the accumulation of phenylalanine (PA) and its metabolic products in the body. Early detection of PKU is achieved through universal neonatal screening of newborns. This enables timely initiation of adequate dietary therapy and contributes to a favorable prognosis. In this article, we present a clinical case of a late diagnosis of the classic form of PKU. The patient was monitored longitudinally from birth until the age of 17 years. A distinctive feature of this case is the diagnosis of phenylketonuria at the age of 2 years. The neonatal period was complicated by perinatal hypoxic-ischemic CNS injury, atelectatic pneumonia, and enteropathic syndrome. These pathological conditions accounted for the early delay in motor and speech development before the child reached 2 years of age. Prior to diagnosis, the child received age-appropriate levels of dietary protein, which played a major role in the development of persistent speech and language impairments and cognitive deficits. Strict dietary therapy initiated after confirmation of the PKU diagnosis resulted in partial compensation of these impairments; however, the early cognitive consequences proved irreversible. Insufficient adherence to treatment subsequently led to the development of episodes of metabolic decompensation. Concomitant somatic pathology, however, did not have a decisive impact on the course of PKU.