Background: We report a case of Epstein-Barr virus-positive NK/T-cell lymphoma primarily presenting with pericardial effusion without nasal lesions. Case: A 63-year-old man with dyspnea and 38°C fever was found in physical examination to have pericardial effusion of 1, 000ml. Tumor cells in effusion showed marked nuclear pleomorphism and variable-sized basophilic cytoplasm with azurophilic granules. Flow cytometoric and immunohistochemical analysis showed tumor cells to be positive for CD2, CD3ε, CD7, CD45RA, CD45RO, CD56, perforin, T-cell intracellular antigen 1, granzyme B, but negative for CD4, CD5, CD8, CD20, CD30, terminal deoxynucleotidyl transferase, and T-cell receptor β. An in situ hybridization study showed positive signals for Epstein-Barr virus encoding small ribonucleic acids on tumor cells. Despite chemotherapy, the patient died of respiratory failure due to disease progression 2 months after admission. No autopsy was done. Conclusion: Based on the above data, we definitively diagnosed this case as extranodal NK/T-cell lymphoma of nasal type. It appeared to be unique in clinical manifestation, primarily affecting the pericardium.