In an attempt to define diagnostic criteria for the differentiation of pleural exudates from transudates, we measured ferritin (FER), interleukin-6 (IL-6), interleukin-8 (IL-8) and tumor necrosis factor-alpha (TNF-alpha) in pleural effusions and blood serum in 84 consecutive patients with pleural effusions of various etiologies. Concentrations of FER, IL-8 and TNF-alpha were significantly higher in serum and pleural effusion in patients with exudates than in patients with transudates. Serum concentrations of IL-6 were not significantly increased in pleural exudate patients (9.78 +/- 17.12 fmol/L) compared to transudate patients (4.05 +/- 2.33 fmol/L), while significant differences were found between pleural exudates and transudates (p < 0.001). Increased levels of FER were found in serum and pleural effusion of cancer patients in comparison to non cancer patients (p < 0.001 and p < 0.001, respectively). Serum concentrations of IL-6, IL-8, and TNF-alpha were not significantly increased in cancer compared to non-cancer patients, while increased concentrations of IL-6 and IL-8 were found in pleural fluid of patients with cancer in comparison to non-cancer patients. Finally/ no statistically significant differences were found in serum and pleural TNF-alpha concentrations among patients with cancer and patients with non-cancer effusion. We conclude that FER, IL-6, IL-8 and TNF-alpha concentrations in pleural effusions are useful markers in differentiating exudates from transudates.