Hematopoietic stem cell transplantation (HSCT) is a potentially curative therapy for various types of high-risk hematologic diseases. It can be categorized into autologous HSCT, using the patient’s own stem cells, and allogeneic HSCT, using donor stem cells. During the treatment, recipients are in an immunocompromised state due to high-dose chemotherapy and immunosuppressive agents, particularly when an allogeneic graft is used. Resetting (as in autologous HSCT) or replacing the immune system (as in allogeneic HSCT) may lead to complications such as infections, endothelial dysfunction, and allogeneic immune responses, which can contribute to the development of severe sepsis or organ dysfunction requiring transfer to the intensive care unit (ICU). Transplant-related complications are classified as early and late complications and are influenced by immunosuppressive therapy, immune reconstitution, and the presence of graft-versus-host disease and its treatment. A thorough understanding of the pathophysiology of these specific complications is important for guiding treatment strategies. Factors associated with ICU mortality are useful for guiding management in high-risk patients.