Acute lymphoblastic leukemia (ALL) is one of the most common malignant diseases of the hematopoiesis system in childhood, characterized by uncontrolled proliferation of a tumor clone from lymphocyte progenitor cells. At the same time, as a result of total immunosuppression, the most common complications are infectious. The purpose of the work: to describe a clinical case of nosocomial pneumonia, formed against the background of chemotherapy relapse of ALL. Materials and methods. A retrospective analysis of the medical records of patient L., 6 years old, who was being treated at the Department of Pediatric Pulmonology of the UKB No. 1 of the Razumovsky SSMU with the main diagnosis: Nosocomial polysegmental pneumonia with obstructive syndrome, acute course. Complications: 1. RF III art., 2. Pulmonary edema, 3. Exudative pleurisy. Background: ALL III phenotype; late bone marrow relapse, isolated (30.12.2022). Results. At the age of 2 (2019) - he made his ALL debut, received induction courses, and achieved remission within the prescribed consolidation period. In 2022, a reinduction course was conducted, and it was removed from the D-register. Relapse of leukemia (28.12.2022) - hospitalized in the V.Ya. Shustov UCB No. 3, 1 block of anti-relapse therapy of ALL in combination with hemotransfusion, massive antibacterial (AB) therapy was performed. On the 23rd day of hospitalization in the hematology clinic, nosocomial 2-sided polysegmental pneumonia was diagnosed, complicated by pulmonary edema, exudative pleurisy, RF III art. On 23.01.2023, the patient was urgently taken to the children’s Intensive care Unit (DORIT) in extremely serious condition UKB No. 1 named after S.R. Peacemakers SSMU, where over the next 15 bed days he received: erythrocyte suspension transfusion, oxygen therapy, combined AB therapy, GCS, bronchodilator and mucolytic therapy, against which the condition improved: symptoms of intoxication, DN were stopped, the inflammatory process in the lungs was resolved. For further treatment of relapse, ALL was transferred to the hematology clinic. Conclusion: The course of chemotherapy for relapse of ALL in the patient provoked a severe infectious complication - nosocomial pneumonia with DN III art., pulmonary edema, pleurisy. Timely diagnosis of pneumonia and intensive therapy in DORIT conditions contributed to a favorable outcome of the infectious process in the lungs. Taking into account the above, the tactics of ALL management were changed - allogeneic transplantation of hematopoietic stem cells from a haploidentical donor (father) was performed, 19.07.2023 - engraftment was achieved.