Fibronectin concentrations both in plasma and pleural effusion were prospectively determined in 60 patients with exudative pleural effusions. Fibronectin concentrations in plasma and pleural fluid in 12 patients with infectious and exudative pleural effusions (mean +/- SD) were 240 +/- 103 and 212 +/- 115 micrograms.mL-1, in 17 patients with primary or metastatic lung carcinoma 242 +/- 104 and 210 +/- 82 micrograms.mL-1, in 13 patients with pleural tuberculosis 246 +/- 77 and 231 +/- 133 micrograms.mL-1, and in 18 patients with confirmed malignant pleural mesothelioma 261 +/- 119 and 276 +/- 188 micrograms.mL-1. There were no significant differences either in the plasma or serum concentrations of fibronectin between groups (p > 0.05). Although pleural fluid fibronectin content appeared to have high specificity (85%), it was found to be an inefficient biological marker for differentiating nonmalignant from malignant pleural effusions due to its low sensitivity (6%).