The aim of this study was to investigate the prevalence of intestinal colonization by major multidrugresistant (MDR) bacteria and fungi in patients with hematological malignancies (HM) and its relationship with subsequent bloodstream infections (BSIs). The study was performed between December 2023 and June 2024 at the University Hospital "Saint Marina", Varna, Bulgaria. A total of 180 HM patients were screened for intestinal colonization by 3rd generation cephalosporin-resistant (3rdGCephR) and carbapenem-resistant (CR) Enterobacterales, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Vancomycin-Resistant Enterococci (VRE) and Candida sp. according to our own protocol. Blood cultures were taken according to clinical indications. Multiplex PCR was used to detect beta-lactamase genes in all invasive Enterobacterales isolates. A total of 100 patients (55.6%) were colonized by one or more of the screened agents. Among these, 88 patients were MDR carriers (48.9%, 88/180) and the highest colonization rates were found for CR Klebsiella pneumoniae (CRKP) (42%), Candida sp. (34%), VRE (25%) and 3rdGCephR Escherichia coli (23%). A total of 29 patients (16.1%; 29/180) developed BSIs, with K. pneumoniae responsible for 44.8%. Of these, 76.9% (10/13) were CR and blaNDM positive isolates. Related BSIs were diagnosed in 57.9% of the MDR carriers with BSIs (11/19). The related BSIs percentage according to the colonizing agent was 21.4% for CRKP (9/42), 7.1% for 3rdGCephR K. pneumoniae (1/14) and 25% for P. aeruginosa (1/4). CRKP colonization was significantly higher among patients with CRKP BSIs than those with non-CRKP BSIs (P < 0.001). The MDR intestinal colonization, specifically by CRKP is an important source for subsequent BSIs in this high-risk patient population.  Kulcsszavak: hematological malignancy, opportunistic pathogens, MDR bacteria, intestinal colonization, bloodstream infections