A clinical case of diagnosis of the onset of leukemia in an 8-year-old child, began with a bone-joint syndrome and was the only one at the beginning of the disease, is presented. This made it much more difficult to make a timely diagnosis. In addition to the severity of the articular syndrome at the onset of the disease, polymorphism of primary clinical manifestations was the reason for the examination of the child in children’s departments of various profiles. The diagnostic search took 4 months, since laboratory changes in the patient were observed later than clinical symptoms. It was the lack of effect from the ongoing therapy of the articular syndrome that made it necessary to exclude acute leukemia. The first manifestation of acute leukemia in this patient was osteoarticular syndrome caused by infiltration of the bone marrow and periosteum. The relative rarity of oncological diseases in children, about 95% of which are represented by acute leukemia, leads to a decrease in the alertness of doctors, both primary care and narrow specialists to these forms of pathology and determines the need for a differentiated approach not only to the existing syndromes, but also their combinations. A multidisciplinary approach to the diagnosis of acute leukemia with the implementation of a comprehensive assessment of the course of the disease allows the patient to be referred to a pediatric hematologist-oncologist in a timely manner.