In children with rheumatic diseases (RD), there is often a need for differential diagnosis with malignant neoplasms (MN). Objective. To study the characteristics of patients hospitalized with a preliminary rheumatic diagnosis who were subsequently diagnosed with MN. Materials and methods. Between 2019 and 2024, 30 children with RD were hospitalized, in whom MN were verified during hospitalization. Clinical, laboratory, and instrumental examinations were conducted, therapy and disease outcomes were analyzed. Results. Among all children with preliminary rheumatic diagnoses, MN were verified in 0,13% (30/23,080). The preliminary diagnosis for 14 (47%) children was juvenile idiopathic arthritis, for 11 (37%) – reactive arthritis, for 2 – osteomyelitis, for 1 (3%) – panniculitis, for 1 (3%) – septic arthritis, and for 1 (3%) – systemic lupus erythematosus. Following the examination, acute leukemia was confirmed in 21 (70%) patients, lymphomas in 5 (17%) children, Ewing sarcoma in 2 (7%), and neuroblastoma in 2 (7%). The most common symptoms and laboratory findings among the patients included arthralgia (87%), arthritis (73%), fever (73%), weakness (57%), osalgia (53%), lymphadenopathy (53%), elevated ESR (90%), increased C-reactive protein (77%), elevated lactate dehydrogenase levels (57%), and anemia (50%). Of the 22 children followed up in the long-term observation, 2 (7%) developed a fatal outcome. Conclusion. Differential diagnosis of MN must be an integral part of the practice of pediatric rheumatologists. © 2025, Pediatria Ltd.. All rights reserved.