BACKGROUND: Extranodal natural killer (NK)/T-cell lymphoma is a subtype of lymphoma that is derived from NK cells. It is considered as an aggressive form of non-Hodgkin's lymphoma because of frequent relapses and resistance to treatment. Relapsed NK/T-cell lymphoma often follows a fulminant course that is refractory to conventional chemotherapy treatment. METHODS: Several patients with extranodal NK/T-cell lymphoma showed long-term survival in spite of frequent relapses. Thus, the medical records of patients diagnosed with extranodal NK/T-cell lymphoma from 1995 to 2007 were reviewed and assessed. RESULTS: Of the 140 cases reviewed, 6 were selected (4.29%). Each of these patients had a minimum of 3 relapses or disease progression during the follow-up period, and their median overall survival was 66 months (range, 42-89 months). They were grouped according to the atypical clinical behavior observed: (1) repeated relapses or progression (> or =3 times) during follow-up; and (2) long-term survival of more than 40 months, as the longest overall survival median was previously considered at approximately 40 months. The clinicopathological and laboratory characteristics of these patients were similar to those of other extranodal NK/T-cell lymphoma patients. However, 5 of the studied cases involved relatively lower expression of the proliferation-related antigen Ki-67 (<40-50%), indicating less proliferative activity. Clinically, they showed delayed relapse for at least 20 months after the initial complete remission. CONCLUSION: Our observations suggest that the clinical behavior of some extranodal NK/T-cell lymphoma patients differs from the typical clinical course.