The relationship between the gut flora and various diseases (obesity, diabetes mellitus, metabolic disorders, allergic and autoimmune diseases, inflammatory bowel diseases, liver failure, infections, certain neuropsychiatric disorders, tumors) has been highlighted in recent years. Depletion of microbiotics inhibits bone marrow healing. Infections and their antibiotic treatment may also affect hematopoiesis. Intestinal flora may also affect the severity of the graft-versus-host disease and may also play a role in the pathogenesis of immunthrombocytopenia through the T-regulator cells. The study summarizes the features of the gut flora, the effects of microbiotics on bone marrow healing, the course of infections, allogeneic bone marrow transplantation, graft-versus-host disease, lymphoma and the results of related research and therapeutic options. The authors briefly discuss the possible linkage between intestinal flora and immunthrombocytopenia and the effectiveness of the immunotherapy of tumors and its effect on the von Willebrand-factor synthesis. They draw attention on the importance of maintaining microbiotics diversity. Orv Hetil. 2019; 160(20): 774-779.