Aim: To compare the course of severe community-acquired pneumonia caused by Kl. pneumonia in patients with acute lymphoblastic and myeloblastic leukaemia.Materials and methods. The study was designed as a retrospective, single-centre, controlled and non-randomized study and included 40 patients. They were divided into two groups: patients with acute myeloblastic leukaemia (22), mortality of 59% (13 patients) and patients with acute lymphoblastic leukaemia (18), mortality of 38% (7 patients).Results and discussion. The combination of acute myeloblastic leukaemia and severe community-acquired pneumonia caused by Kl. pneumonia showed a higher mortality rate, a longer duration of respiratory support, as well as a greater frequency of involvement of organs and systems in the infectious process than in acute lymphoblastic leukaemia.Conclusions: The development of pneumonia caused by Kl. pneumonia in patients with acute myeloblastic leukaemia is characterized by a more severe course. SOFA, APACHE-II, and PSI/PORT scales have limited predictive ability in patients with acute leukaemia and severe community-acquired pneumonia caused by Kl. Pneumoniae.