BACKGROUND The alteration of Th1 and Th2 cytokine levels is the subject of controversy in pleural effusions caused by malignancy, a situation that favors a Th2 immune response. OBJECTIVE To examine the different levels of IL-4 and IL-10 (Th2 cytokines), and IL-2 and interferon-γ (IFN-γ) (Th1 cytokines) in malignant and non-malignant pleural effusions. METHODS The cytokine levels in pleural fluid of 62 patients with malignant pleural effusion (44 with lung cancer and 18 with extrathoracic tumors), 8 with tuberculous and 8 with congestive heart failure pleural effusion were analyzed using enzyme-linked immunosorbent assays. RESULTS IL-2 was below the detectable concentration of the assay. A significant decrease in IFN-γ level was observed in malignant but not in congestive heart failure cases compared to tuberculous cases. IL-10 levels were higher in malignant and tuberculous pleural effusions than in congestive heart failure pleural effusions, however, this difference did not reach the significant level. IL-4 levels were also increased non-significantly in lung cancer pleural effusions compared to the other groups. CONCLUSIONS Our results show a wide variation in IL-4, IL-10, and IFN-γ levels in malignant pleural effusions, a pattern which was not convincing enough to differentiate the cause of effusion.