Background. Marshall syndrome, also known as the PFAPA syndrome, is an autoinflammatory disease characterized by periodic fever, aphthous stomatitis, pharyngitis, and cervical adenitis. This widely common pediatric autoimmune disease often remains undiagnosed due to a lack of knowledge about its clinical features. Therefore, it can be assumed that Marshall syndrome is much more common than it is diagnosed. We describe a clinical case of Marshall syndrome in a five-year-old boy by retrospectively analyzing the patient’s anamnesis, the course of the disease, the results of laboratory and instrumental studies. The treatment approach is also presented. Despite the complexity of this pathology, a positive result in the treatment of the child was achieved. Clinical case description. A five-year-old boy was admitted to the Cardio-Rheumatology Department of the Krasnodar Krai Children’s Regional Clinical Hospital for examination and clarification of the diagnosis. The patient’s parents complained of recurrent episodes of fever. During such episodes, the child’s blood tests revealed leukocytosis, increased erythrocyte sedimentation rate (ESR), and increased C-reactive protein levels. After the end of the episode, all indicators returned to normal levels. The anamnesis showed that, during the previous three months, the boy had been repeatedly hospitalized with various diseases. The preliminary diagnosis was “Juvenile arthritis, systemic variant. Autoinflammatory disease?”. Laboratory studies revealed antineutrophil cytoplasmic antibodies to proteinase-3 (cANCA), to myeloperoxidase (pANCA), and antinuclear antibodies (ANA) in a titer of 1/160. For differential diagnosis, genetic screening for familial Mediterranean fever was performed; however, no mutations in exons two, three, five, and ten of the MEFV gene were found. As a result, the child was diagnosed with “Autoinflammatory disease. Marshall syndrome.” The prescribed therapy with glucocorticosteroid drugs and immunosuppressants produced a positive effect. After recovery, the boy was discharged from the hospital under the supervision of a pediatrician at the place of residence. Conclusion. Difficulties in the treatment of such patients lie, first of all, in the verification of the diagnosis. Indeed, at early stages, the clinical symptoms of monogenic forms of autoinflammatory diseases and other diseases may be similar to Marshall syndrome. As a result, the patients are subjected to unnecessary massive antimicrobial therapy, resulting in deterioration of their quality of life for many years. Untimely diagnosis does not result in an unfavorable outcome or disability due to the benign clinical course of the disease.