Extranodal NK/T-cell lymphoma (NTCL) is characterized by clinical heterogeneity based on clinical prognostic factors and survival outcome. NTCL subsets are classified as upper aerodigestive tract (UAT) NTCL or non-UAT NTCL; non-UAT has pathologic similarity to UAT-NTCL but is a clinically distinct subtype. Due to the clinical heterogeneity of NTCL, optimal treatment modalities and prognostic factors have been difficult to determine. Ann Arbor staging for lymphomas and the International Prognostic Index (IPI) have been used to predict prognosis for UAT-NTCL; however, local tumor invasiveness (bony invasion or perforation or invasion of the overlying skin) is the most significant factor for poor outcomes in localized UAT-NTCL. Thus, a new staging system is proposed: limited disease (stage I/II UAT-NTCL without local tumor invasiveness) and extensive disease (stage I/II with local invasiveness or stage III/IV disease of UAT NTCL, and non-UAT NTCL) based on treatment outcomes. NTCL is resistant to anthracycline-based chemotherapy, whereas non-anthracycline combination chemotherapy (such as ifosfamide, methotrexate, etoposide, and prednisolone) has an activity against NTCL as either a front-line or as a second-line treatment. The effectiveness of radiotherapy is evident in limited disease, but questionable in extensive disease. (Cancer Sci 2009; 100: 2242–2248)