Introduction. Lymphomas account for about 12% of malignant tumors in children, and anaplastic lymphoma for 10–20% of Hodgkin’s and non-Hodgkin’s lymphomas. Clinical symptoms associated with malignant tumors of the lymphatic system are not specific. Diagnosis of these tumors is particularly difficult in the absence of a visible tumor or enlarged peripheral lymph nodes, especially when the symptoms may suggest other, far more common diseases such as infections. Extensive clinical diagnostic procedures, including the exploratory laparotomy, intensive symptomatic treatment and antibiotic therapy do not explain the nature of the disease, do not improve the condition of a patient and lead to the death of sick children. In these cases only an autopsy and histopathological examinations demonstrate the presence of anaplastic large T-cell lymphoma’s infiltrates of internal organs, bone marrow and lymph nodes. Aim. The aim of this study was to demonstrate that in the diseases of children in which it is difficult to establish a definite clinical diagnosis and an intensive antibiotic therapy does not cause any improvement, a neoplastic disease should be always taken into consideration. Materials and methods. Analysis of the histo-clinical picture of a disease of a child who died due to ALK+ anaplastic large T-cell lymphoma. Diagnostic difficulties resulted in not establishing a clinical diagnosis. Despite conservative treatment, surgical procedure and an intensive antibiotic therapy, the death occurred. The diagnosis was established post-mortem on the basis of immunohistochemical tests: LCA, CD30, CD43, Granzyme B, ALK, CD20, CD3, MPO and Ki67. Case study and results. A 14-year-old boy went to the doctor because of abdominal pain and fever. After week-long treatment with antibiotic (Duomox, Astellas Pharma) acute symptoms subsided, but then relapsed after a month and the boy was admitted to the surgery department. Biochemical studies showed increased levels of inflammatory process markers and aminotransferases. Physical examination revealed positive peritoneal signs. With the suspicion of acute Meckel’s diverticulitis, laparotomy and appendectomy were performed. During the surgery, a significantly enlarged right lobe of liver was found. Antibiotic treatment was administered and after a few days following the surgery the symptoms subsided. On the 5th day after the surgery the patient’s condition deteriorated and on the 8th day he died. The diagnosis was established on the basis of autopsy: ALK+ anaplastic large T-cell lymphoma (LCA+, CD30+, CD43+, ALK+, Granzyme B+, Ki67+ in 85% of cells). Discussion and Conclusions. Cooperation and efficient communication between the clinician and pathologist are important and necessary in all cases when it is difficult to establish a correct and rapid diagnosis. If in an inflammatory disease the patient’s condition is deteriorating despite an intensive antibiotic therapy, the neoplastic disease should be always taken into consideration and the diagnostics should focus on searching for a tumor.