We report a patient with a systemic cytomegalovirus (CMV) infection, which occurred during postoperative chemoradiotherapy for a malignant astrocytoma. To our knowledge, there is no report that is especially focused on the association with a CMV infection. Interstitial pneumonia and gastrointestinal bleeding, which developed suddenly during postoperative chemoradiotherapy, resulted in the patient's death. A histopathological examination of the postmortem specimens revealed numerous "owl's eye" cells containing intranuclear inclusion bodies, which were identified as CMV by immunohistochemical examination and in situ hybridization. The premortem diagnosis of CMV infection is usually difficult, because an anti-CMV titer can be nonspecifically elevated. With immunohistochemical examination and in situ hybridization, CMV in excretory or biopsy specimens can be identified and the diagnosis of CMV infection can be established. When serious pneumonia or massive gastrointestinal bleeding occurs during postoperative chemoradiotherapy, the differential diagnosis should include the possibility of CMV infection and we recommend an immunohistochemical examination and in situ hybridization for the detection of CMV.