Aim. To demonstrate a clinical case of orphan hereditary disease from the group of lysosomal storage diseases - Pompe disease (glycogenosis type II) in a five-year child. Materials and methods. Anamnestic data, clinical and laboratory parameters and treatment of a five-year child with Pompe disease. The patient was observed in the gastroenterological department of the Children’s Regional Clinical Hospital. Results. Child admitted with complaints of muscle weakness, difficulty climbing stairs, rare headaches. According to his history, his mother turned to the pediatrician about the prolonged course of a respiratory infection. An outpatient examination revealed a significant increase in the levels of alanine aminotransferase and aspartate aminotransferase. Viral hepatitis was excluded. The child was hospitalized in a hospital for further examination with hepatitis, unspecified etiology (non-infectious). Examination of the child revealed a lag in physical development, muscle hypotension, hepatomegaly, liver dysfunction, and increased total creatine phosphocanase and IgE. A decrease in the activity of acid α-1,4-glucosidase in the spot of dried blood was determined according to tandem mass spectrometry and gene mutations GAA DNA diagnostic method. Diagnosed with Pompe disease (glycogenosis type II). Myopathic syndrome. The treatment with the genetic engineering enzyme-substituting drug Mayozayme was started at a dose of 20 mg/kg, intravenously, once every two weeks. Conclusion. Pompe disease is a rare pathology characterized by a low frequency of prevalence and polymorphism of clinical manifestations, complicating the diagnosis. Timely diagnosis of the disease and the earliest possible appointment of pathogenetic therapy are required to improve patients’ quality of life, slow down the progression of the disease, and prevent the development of life-threatening complications.