Although the testis is not uncommonly involved during the course of disease in both nasal and non-nasal extranodal NK/T-cell lymphoma (ENKTCL), only a few cases presenting initially with a testicular mass have been previously reported. These have been documented as case reports, rather than as study series. Because of its rarity, the clinicopathologic features and the prognosis of de novo testicular ENKTCL have not been well characterized. Clinicopathologic features of 21 cases of de novo testicular ENKTCL from 3 institutions in China were retrospectively analyzed with review of an additional 18 cases from the literature. De novo testicular ENKTCL accounted for 0.72% (21/2906) of all ENKTCL during the study period. The median age of patients with de novo testicular ENKTCL was 45 years (range, 21 to 79 y). Most (90.9%) cases occurred in Asians. All patients initially presented with testicular swelling and most (91.9%) had unilateral testicular masses. The majority (73.0%) of patients presented at Ann Arbor stage I/II. Expression of CD56 was found in 92.1% (35/38) of the available cases. Interestingly, aberrant expression of CD20 was found in the tumor cells in 10.3% (4/39) of cases. The majority of patients with follow-up data (24/30, 80%) had extratesticular involvement during the follow-up period (median follow-up, 6 months; range, 0.5 to 87 mo). Preferential sites of extratesticular involvement included lymph nodes, skin, contralateral testis, bone marrow, spleen, adrenal gland, and central nervous system. Of the 30 patients with survival data, 70% (22/30) of patients died of the disease. The 2-year overall survival of patients with de novo testicular ENKTCL was 23%, and the median survival was 9.5 months. Patients that presented with B symptoms showed a trend toward inferior overall survival (P = 0.095). No statistical significance was found between patients with stage I/II and stage III/IV (P=0.783). De novo testicular ENKTCL tends to disseminate early, shows extremely poor outcome, and should be recognized as a highly aggressive form of ENKTCL. A portion of cases show aberrant expression of CD20, and accurate diagnosis as well as timely and optimal treatment are very important.