Background: Extranodal natural killer (NK)/T-cell lymphoma (ENKL) is a heterogeneous entity with poor survival, requiring risk stratification in affected patients. We proposed absolute lymphocyte count (ALC) as a new prognostic factor in ENKL. Patients and methods: We retrospectively analyzed 128 patients newly diagnosed with ENKL. Independent prognostic factors of survival were determined by Cox regression analysis. Results: Patients with low ALC (< 1.0 x 10(9)/l) at diagnosis tended to have more adverse clinical features. Patients with high ALC (>= 1.0 x 10(9)/l) at diagnosis had better overall survival (OS; P < 0.0001) and progression-free survival (PFS; P< 0.0001), and achieved higher complete remission rates (P= 0.001). Multivariate analysis with known prognostic factors showed that ALC, B symptoms and advanced stage were independent predictors for OS and PFS. Using the International Prognostic Index, Prognostic Index for Peripheral T-cell lymphoma unspecified, or Korean Prognostic Index for nasal NK/T-cell lymphoma, the majority of patients were in the low-risk category (with no or one adverse factor). ALC was helpful to differentiate the low-risk patients with different survival outcomes (P < 0.0001). Conclusions: Our data suggest that ALC at diagnosis is a novel, powerful predictor of prognosis in ENKL. Immune status at diagnosis might have an important influence on survival in patients with ENKL.