To evaluate the frequency of bone marrow involvement by nasal-type NK/T cell lymphoma, we retrospectively studied biopsy specimens from 40 patients by EBV in situ hybridization (ISH). Three patients had marrow involvement at initial diagnosis (7.5%). In one patient (1/40, 2.5%), the disease in bone marrow was recognized by routine morphological assessment, while two other patients had minimal involvement of lymphoma cells which was recognized only by EBV in situ hybridization (2/40, 5%). Two patients had a disseminated disease at diagnosis and died 6 days and 214 days after diagnosis. One patient had diffuse colonic lesion and died 82 days later. In conclusion, marrow involvement in nasal NK/T cell lymphoma is infrequent at initial diagnosis, and EBV ISH is a useful technique for identifying the minor subgroup of patients which have easily overlooked neoplastic involvement.