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Interleukin (IL)-1alpha, IL-6, and tumor necrosis factor (TNF)-alpha were measured in pleural fluid from 57 patients with pleural effusion in order to evaluate the diagnostic utility of these cytokines. We studied 20 patients with malignant pleural effusion, 11 patients with parapneumonic pleural effusion, 9 patients with tuberculous pleural effusion, and 17 patients with transudative pleural effusion. Cytokines were measured by radioimmunoassay.University tertiary hospital.The mean values of the three cytokines measured in pleural fluid or in serum were significantly higher in patients with exudates than with transudates (p0.05). The ratio of IL-6 in pleural fluid to serum was significantly higher in exudates than in transudates (p0.05). The level of IL-6 in pleural fluid was significantly higher in tuberculous than malignant (p0.007) or parapneumonic pleural effusions (p0.04). No significant difference between the three types of exudates was found in pleural fluid levels of IL-1alpha or TNF-alpha.Serum levels of IL-1alpha, TNF-alpha, and in particular IL-6 can distinguish exudates from transudates, while pleural fluid IL-6 levels could be useful as an additional marker in the differential diagnosis of tuberculous, malignant, and parapneumonic exudates. Finally, our results suggest that there is local cytokine production in exudative pleural effusions.