Sinonasal natural killer (NK)/T-cell or T-cell lymphoma behaves quite differently from other lymphomas. The objective of this study was to investigate clinical features, treatment outcomes, and failure patterns in patients with this type of sinonasal lymphoma. From September, 1977 to December, 2000, 77 patients with sinonasal NK/T-cell lymphoma or T-cell lymphoma who had received radiotherapy (R/T), chemotherapy (C/T), or both (R/T and C/T) were evaluated retrospectively. Fifty-six patients (73%) had locoregional disease only, and 21 patients (27%) had systemic involvement. Forty-four patients (57%) achieved a complete remission (CR). The 5-year overall survival rate was 36% (median follow-up, 89 months). Achievement of CR was the only prognostic factor for survival in multivariate analysis. Among patients with locoregional disease, the CR rate was 63%, and the 5-year overall survival rate was 42%. Combined R/T and C/T or R/T alone resulted in better survival compared with C/T alone (5-year survival rates, 59%, 50%, and 15%, respectively; P = 0.01). Incidences of locoregional and systemic failure were 43% and 30%, respectively. Outcome was dismal for patients with systemic disease, with a CR rate of 43% and a 5-year survival rate of 25%. Only 2 of 21 patients had sustained remissions. The locoregional and systemic failure rates were 57% and 71%, respectively. Treatment outcomes were unsatisfactory for patients with locoregional and systemic sinonasal NK/T-cell or T-cell lymphoma. R/T could not control locoregional disease satisfactorily, and C/T was unable to eradicate systemic disease in many patients. High-dose therapy may be worth studying in these patients. New treatments should be investigated to increase remission rates, prevent failure, and improve survival. Copyright 2003 American Cancer Society.