This report describes a 41-year-old patient, who developed herpes simplex virus type 2 (HSV-2)-hepatitis after umbilical cord blood transplantation (CBT). The patient had received allogeneic bone marrow transplantation from an unrelated donor for acute myeloid leukemia (AML) not in remission. AML relapsed 18 months after the first transplantation, and CBT was performed. AML relapsed again 5 months later and the patient was given chemotherapy. Although there was no active chronic graft-versus-host disease, liver dysfunction appeared, and one week later, progressed to acute liver failure. Viral screening of blood by PCR including hepatitis B and C viruses, human immunodeficiency virus, Epstein-Barr virus, cytomegalovirus, herpes simplex virus type 1 and HSV-2 revealed elevation of HSV-2 (2.34 × 10⁴ copies/ml). We diagnosed the patient as having HSV-2 acute hepatitis, and initiated treatment with antiviral drugs (acyclovir, foscarnet) and plasma exchange. However, liver functions deteriorated rapidly, and the patient died on day 6 after the onset of acute liver failure. Although HSV hepatitis is very rare after allogeneic stem cell transplantation, it is rapidly progressive and associated with a high mortality rate. Thus, early diagnosis with prompt antiviral intervention is recommended.