Fever is a common complaint in children with lymphadenopathy. Reactive lymphadenopathy is defined as the presence of one or more lymph nodes with a diameter of more than 1.5-2.0 cm. A thorough diagnosis of lymphadenitis accompanied by hyperthermic syndrome is necessary. Purpose - is to determine the diagnostic algorithm of actions based on the example of a clinical case of fever of unclear origin. Clinical case. The peculiarity of this case is a prolonged hyperthermic syndrome with lymphadenitis against the background of helminthic invasion (toxocarosis). During her stay in the hospital, the girl’s condition was serious due to hyperthermic syndrome and lymphadenopathy. The diagnostic algorithm for this condition in children is proposed. During the examination, a diagnosis of Reactive cervical lymphadenopathy of infectious-allergic genesis; concomitant - «Parasitic invasion is mixed (toxocariasis, ascariasis). Fever» was established. The child’s condition upon admission was severe, due to intoxication and hyperthermic syndrome. The treatment was effective, the patient’s condition improved during the hospitalization. But establishing a diagnosis required the cooperation of doctors of many specialties and a large number of additional examinations. Conclusions. Therefore, it is necessary to form the alertness of pediatricians to toxocarosis and increase the level of knowledge about the correct diagnostic algorithm in such cases. Informing health care professionals about such lymphadenopathy and helminthic infestation in children should help practitioners distinguish them from febrile reactions or other conditions and improve treatment outcomes. The research was carried out in accordance with the principles of the Declaration of Helsinki. Informed consent of the child’s parents was obtained for the research. No conflict of interests was declared by the authors.