Objective: Pleural effusion is frequently encountered in patients with lung cancer. Malignant-benign differentiation of the fluid is very important for treatment decision because malignant fluid is considered as the inoperability criterion. However, this distinction is not clinically feasible and may require a cytological examination of the fluid via invasive procedures. The aim of this study was to determine whether there was any difference between laboratory results of malignant and benign pleural fluids. Methods: We retrospectively evaluated 135 patients with cytologically diagnosed lung cancer and underwent benign-malignant differentiation of pleural effusion. Benign and malignant groups were compared in terms of fluid biochemistry, blood gas and cell count. Results: One hundred four patients were male, 31 were female and the mean age was 63.5±11.4 years. Histologically adenocarcinoma was determined as the most common (56%). Right pleural effusion was present in 58.5% of the patients. Malignant effusion rate was higher in females (malign/benign; female: 21/10, male: 48/56). Albumin, protein, erythrocyte count (RBC) and hematocrit (HCT) values in pleural fluid were higher in the malignant group (p=0.001, p=0.018, p=0.009 and, p=0.016, respectively). Cut-off value for albumin: 2.85 and odds ratio (OR): 2.02; for HCT 4.7 and OR: 6.25; for RBC 300 and OR: 6.25; for protein 4.45 and OR: 2.08.