Epstein-Barr virus (EBV) infection is highly associated with specific subtypes of malignant lymphoma. In our previous report on nodal malignant lymphoma in Thailand, we found that 64% of classical Hodgkin's lymphoma (cHL), 51% of non-Hodgkin's lymphoma, T-cell (NHL-T), and 13% of non-Hodgkin's lymphoma, B-cell (NHL-B) were EBV-related. In the present research, we conducted a retrospective study of primary extranodal non-Hodgkin's lymphoma of the sinonasal tract (e-NHL-ST) and primary extranodal non-Hodgkin's lymphoma of the nasopharynx (e-NHL-NP) in Southern Thailand, between 1997 and 2004. EBV-encoded RNA (EBER) expression by in situ hybridization was performed in all cases and a T-cell receptor (TCR)-g gene rearrangement study was performed in NHL-T cases. There were 18 cases of e-NHL-ST and 42 cases of e-NHL-NP detected by histologic and immunohistochemistry examinations. The percentages of e-NHL-ST and e-NHL-NP as compared to nodal malignant lymphoma were 3.7% and 6.8%, respectively. Sixteen cases (88.9%) of e-NHL-ST and 7 cases (16.7%) of e-NHL-NP were NHL-T, and the remainder were NHL-B. All of the NHL-T cases in both sites were EBER-positive. Two (5.4%) of the NHL-B cases in the nasopharynx showed EBER positive. Monoclonal bands of the TCR-gamma gene were detected in 71.4% of the extranodal NK/T-cell lymphomas, nasal type, patients; 50.0% of peripheral T-cell lymphoma, unspecified, patients; and one case of angioimmunoblastic T-cell lymphoma. This study indicates a very strong association of NHL-T in the sinonasal tract or nasopharynx with EBV infection, the link apparently being weaker in NHL-B patients. The study also indicates that most cases of extranodal NK/T-cell lymphoma, nasal type, are not the germline configuration of the TCR genes.