Purpose To elucidate the management and outcomes of patients with extranodal natural killer/T-cell lymphoma, nasal type (ENKL), who were diagnosed between 2000 and 2013 in Japan. Patients and Methods Data from 358 patients with ENKL diagnosed between 2000 and 2013 from 31 institutes were retrospectively analyzed. Results Patients' median age was 58 years, and 257 (72%) had localized disease. The most common first-line treatment was radiotherapy with dexamethasone, etoposide, ifosfamide, and carboplatin (RT-DeVIC) (66%) for localized ENKL and L-asparaginase-containing chemotherapy (30%) for advanced ENKL. With a median follow-up of 5.8 years, overall survival (OS) rates at 5 years for localized and advanced ENKL were 68% and 24%, respectively. The prognostic index of natural killer lymphoma was validated in our study, although only 4% of patients with localized ENKL were classified as high risk. With a median follow-up of 5.6 years, OS and progression-free survival at 5 years in the 150 patients who received RT-DeVIC in clinical practice were 72% (95% CI, 63% to 78%) and 61% (95% CI, 52% to 69%), respectively. Toxicities of RT-DeVIC were comparable to those in a previous trial. Multivariate analysis in patients with localized ENKL who received RT-DeVIC identified elevated soluble interleukin-2 receptor as an independent predictive factor for worse OS and progression-free survival (adjusted hazard ratios, 2.28 and 2.46; 95% CI, 1.24 to 4.23 and 1.42 to 4.28; P = .008 and .0014, respectively). Conclusion Favorable OS in response to new treatments was demonstrated in a large number of patients. Improved treatment approaches are needed for localized ENKL exhibiting elevated pretreatment soluble interleukin-2 receptor.