We report the autopsy case of a 34-year-old Japanese man with a nasal natural killer (NK)-cell/T-cell lymphoma. The patient developed the disease at 32 years of age, and a biopsy of the nasopharynx revealed pleomorphic lymphoma cell proliferation. Radiotherapy was performed, but the patient eventually died of respiratory failure. After radiotherapy, no histologic evidence of malignancy was obtained with biopsy materials featuring lymphocytic infiltration. Autopsy studies, including in situ hybridization for Epstein-Barr virus-encoded RNA, revealed generalized infiltration of normal lymphocyte-like, UCHL-1-positive, and Epstein-Barr virus-encoded RNA-positive lymphoma cells. Monoclonal proliferation of the Epstein-Barr virus-carrying cells was verified by means of Southern blot analysis. Retrospectively, we concluded that the normal lymphocyte-like presentation of the lymphoma cells, probably influenced by radiotherapy, prevented pathologists from recognizing the lymphoma. The utility of in situ hybridization for Epstein-Barr virus-encoded RNA in identification of tumor cells is emphasized with respect to the present case.