In the article on the example of phenylketonuria, modern aspects of the prevention of rare diseases are considered. Phenylketonuria is a hereditary disease, in the absence of treatment characterized by progressive mental retardation, epilepsy, severe dermatitis, but with early diagnosis and diet therapy, clinical manifestations of the disease can be avoided. Primary prophylaxis of phenylketonuria is aimed at preventing the appearance of a sick child in the family and is most effectively carried out with the help of medical genetic counseling. Secondary prophylaxis consists in prenatal diagnosis of pathology, as well as neonatal screening for phenylketonuria. Tertiary prevention includes treatment, abilitation and rehabilitation of patients. In practice, these approaches are closely interrelated, which is reflected in the article. The role of pregravid preparation of women with phenylketonuria to prevent the syndrome of «maternal phenylketonuria» in a future child is underlined.