Natural killer/T-cell lymphoma is rare and aggressive, with poor outcome. Optimal treatment remains unclear. A novel regimen dexamethasone, methotrexate, ifosfamide, l-asparaginase, and etoposide (SMILE) showed promise in phase 1/2 studies with restrictive recruitment criteria. To define the general applicability of SMILE, 43 newly diagnosed and 44 relapsed/refractory patients (nasal, N = 60, nonnasal, N = 21; disseminated, N = 6; male, N = 59; female, N = 28) at a median age of 51 years (23-83 years) were treated. Poor-risk factors included stage III/IV disease (56%), international prognostic index of 3 to 5 (43%), and Korean prognostic scores of 3 to 4 (41%). A median of 3 (0-6; total = 315) courses of SMILE were administered. Significant toxicities included grade 3/4 neutropenia (N = 57; 5 sepsis-related deaths); grade 3/4 thrombocytopenia (N = 36); and nephrotoxicity (N = 15; 1 acute renal failure and death). Interim analysis after 2 to 3 cycles showed complete remission rate of 56%, partial remission rate of 22%, giving an overall response rate of 78%. On treatment completion, the overall-response rate became 81% (complete remission = 66%, partial remission = 15%). Response rates were similar for newly diagnosed or relapsed/refractory patients. At a median follow-up of 31 months (1-84 months), the 5-year overall survival was 50% and 4-year disease-free-survival was 64%. Multivariate analysis showed that international prognostic index was the most significant factor impacting on outcome and survivals.