The clinical observation of a boy with protein-losing enteropathy after the Fontan operation is presented. The boy, S.M., 2 years old, was admitted with complaints of abdominal pain, bloating, enlarged abdomen, frequent watery stools, and peripheral edema in the arms and legs. The child was diagnosed with a congenital heart defect at birth, for which Fontan surgery was performed. One month later, diarrhea and swelling appeared on the face, arms, and legs. Edema was temporarily relieved by symptomatic therapy. At the age of 2, the child was hospitalized in the gastroenterology department of our center. Examinations revealed low levels of total protein and albumin, and an increase in alpha-1 antitrypsin values. The diagnosis of protein-losing enteropathy after the Fontan operation was established. The child was prescribed a diet enriched with medium-chain triglycerides, spironolactone, and prednisolone. After 6 months of follow-up, no edema was observed, and the child’s overall condition was satisfactory. However, over the past year until the present, the boy has experienced slight swelling in the arms, legs, and face, requiring periodic albumin transfusions. Fontan operation and protein-losing enteropathy represent different aspects of medicine; however, sometimes one diagnosis becomes a consequence of another. As a collective of authors, we conducted the research, processed the data, obtained results, and we believe that we are correct. We affirm that the responsibility for the results and activity in writing the material remains with the authors.