Several prognostic factors have been studied in NSCLC, although it is unknown which is most useful. In this study, we aimed to investigate whether pre-treatment serum albumin level has prognostic value in patients with Stage IIIB NSCLC. This cross-sectional study included a total of 204 patients with Stage IIIB NSCLC who met the inclusion criteria. Pre-treatment serum albumin levels and demographic, clinical, and histological characteristics, as well as laboratory variables were recorded. A cut-off value was defined for serum albumin level and the patients were stratified into four groups on thios basis. The majority of the patients was males and smokers, with a history of weight loss, and squamous histological type of lung cancer. The mean serum albumin level was 3.2±1.7 g/dL (range, 2.11-4.36 g/dL). A cut-off value 3.11 g/dL was set and among the patients with a lower level, 68% had adenocarcinoma and 82% were smokers. The patients with low serum albumin levels had a lower response rate to e first-line chemotherapy with a shorter progression-free survival and overall survival. Multivariate analysis showed that low serum albumin level was an independent poor prognostic factor for NSCLC. This study results suggest that low serum albumin level is an independent poor prognostic factor in patients with Stage IIIB NSCLC, associated with reduction in the response rate to first-line therapy and survival rates.