This clinical case presents a combined herpes virus infection (cytomegalovirus and HSV-2) in a 55-year-old female patient after allogeneic hematopoietic stem cell transplantation, which manifested as colitis. Despite prophylactic use of acyclovir, viral reactivation occurred, indicating the need for long-term antiviral prophylaxis. The diagnostic process was complicated by non-specific clinical presentation and negative results of polymerase chain reaction and serological tests, which required morphological and immunohistochemical examination of biopsy specimens. This clinical case demonstrates the peculiarities of diagnosis and treatment of herpes virus infections in immunocompromised patients after bone marrow transplantation. © Барам Д.В., Крысюк О.Б., 2025 г.