Background. In the international clinical practice there have been occasional reports of phenylketonuria (PKU) and cystic fibrosis (CF) found simultaneously in the same patient. Both PKU and CF are the inherited disorders characterized by autosomal recessive type of inheritance. Currently the combination of two or more inherited disorders in one patient is considered to be a clinical rarity.Case description. This is a clinical case of two genetic disorders, CF and PKU, combined in a 5-year old patient who had been followed up since birth. Owing to implementation of neonatal screening for inherited and congenital diseases into clinical practice, during the first month of life the infant was diagnosed with CF (diagnostically significant elevation of immunoreactive trypsin [IRT] at the initial [163.2 ng/mL] and repeat testing on day 21 of life [138.7 ng/mL]) and PKU (phenylalanine [PA] level 15.9 mg/dL). Both disorders have been confirmed by genetic tests, i.e., homozygous DelF508 mutation was found in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and P281L mutation in the phenylalanine hydroxylase (PAH) gene was also present in homozygous state. Child’s parents strictly adhered to dietary and treatment recommendations. By the age of 5 years the child developed symptoms of neurological disorder and disorder of the respiratory system, cognitive impairment and delay in speech development, subclinical epileptiform activity with high risk of epilepsy, and chronic inflammation of the respiratory tract.Conclusion. This case report demonstrates the important role of neonatal screening in early diagnosis and timely start of therapy, and underscores the importance of continuous medication in such genetic disorders as CF and PKU. On the whole, such approach brings about a relative preservation of functioning of the most affected organs and systems. By the age of 5 years the child does not form bronchiectases, shows no signs of chronic hypoxia, nutritional deficiency or pronounced neurologic deficit, and is at low risk for the development of autism spectrum disorder. At the same time, the larger scale and longer-term observations are required in order to make the unequivocal conclusions about the prognosis of these diseases under conditions of modern-day medical follow-up.