Natural killer (NK) cells have been implied in the resistance against certain tumors and virally-infected cells. The prognostic significance of tumor infiltrating NK cells in primary squamous cell lung carcinoma (SqCLC) has not been fully studied. Fifty patients with primary SqCLC were evaluated for the presence of tumor infiltrating natural killer cells subset CD57 (TINK) after surgery. None of them received adjuvant therapy. Immunohistochemical studies of surgery pieces were performed by using the monoclonal antibody CD57. The number of TINK cells was counted by using a MICRON image analyzer. The total area studied for each tumor was of 1 cm(2). In this area, 50 intratumoral fields of 0.173 mm(2) were selected. The reference value used was the median (five TINK cells/field) of all tumors analyzed. After a minimum follow-up of 2 years the Kaplan-Meier method was used to obtain survival curves. Multivariate analysis were performed by using the Cox regression model. The survival was significantly better in patients with more than five TINK cells/field (Logrank P=0.0317). According to TNM classification, in those patients screened as stage IB (37 patients) the differences in survival were significantly higher (Logrank P=0.0016). In the multivariate analysis including TNM (surgical-pathologic stage), age, and endoscopy localization, the risk of death in patients with less than five TINK cells/field was 2.50 fold higher (CI 95%; range 1.07-5.85) than in those patients with more than five TINK cells/field. These results show that TINK cells appear to be a prognostic factor in the survival of patients with SqCLC. The possible antitumoral role of these cells in SqCLC is discussed.