A 25-year-old Japanese man was suffering from high fever, sore throat, arthralgia, and macular salmon-pink eruption. The superficial lymph node was not palpable, and computed tomographic scans from the neck to pelvis demonstrated hepatosplenomegaly without apparent lymphadenopathy. Therefore, the possibility of malignant lymphoma was considered to be extremely low. Serology for Epstein Barr virus (EBV) and cytomegalovirus showed a postinfectious state, and blood culture was negative. Serum rheumatoid factor and antinuclear antibody were negative. Leukocytopenia (2.4 x 10(3)/mul) was observed, and thus a diagnosis of adult-onset Still's disease (AOSD) with hemophagocytic syndrome (HPS) was made. Fifty-five milligrams of prednisolone daily improved his symptoms and leukocytopenia promptly, but high fever with severe and progressive thrombocytopenia occurred 12 days later. Bone marrow aspiration revealed the presence of lymphoma cells and hemophagocytosis, and the CD45 gating analysis showed expanding population of CD2(+), CD3(-), and CD56(+) cells. Further, mucosal ulceration in the nasal cavity was detected. Therefore, a diagnosis of extranodal natural killer (NK)/T-cell lymphoma, nasal type, concomitant with HPS was made, and treatment with dexamethasone, etoposide, ifosfamide, carboplatin (DeVIC) regimen ameliorated his symptoms and platelet transfusion dependency. Later, a high titer of serum EBV-DNA was detected, which supported the diagnosis. Diagnosing AOSD, extranodal presentation of malignant lymphoma such as extranodal NK/T-cell lymphoma, nasal type, should be carefully considered.