Brucellosis is one of the most common zoonotic diseases in the world. The epidemiological situation of brucellosis in the Russian Federation is characterized as unfavorable. The problem of brucellosis remains of particular relevance in regions with developed animal husbandry. Many different clinical manifestations and the absence of specific symptoms of brucellosis make it difficult to diagnose this disease. The clinical case presented in this paper illustrates the delayed diagnosis of brucellosis in a 10-year-old patient with fever, enlarged lymph nodes, liver and spleen. The defects in the provision of medical care were based on the lack of alertness of doctors regarding brucellosis, insufficient interpretation of epidemiological history data and incorrect assessment of the totality of clinical syndromes of the disease and laboratory parameters. As a result, brucellosis was diagnosed only six months after the onset of the disease. Subsequent etiotropic therapy led to stabilization of the patient's condition, who was discharged from the hospital with recommendations under the supervision of an infectious disease specialist.