The concentration of ferritin was measured in the pleural fluid of 108 patients with pleural effusions. In all groups of patients the ferritin concentration was higher in pleural fluid than in serum. The greatest differences, with up to 100 times more ferritin in the pleural fluid, were found for patients with rheumatoid pleurisy, malignant effusions, and empyema. In patients with non-malignant inflammatory pleural effusions the concentration of ferritin in pleural fluid correlated significantly with other pleural fluid indices of inflammation: there was a positive correlation with lactate dehydrogenase activity and a negative correlation with concentrations of glucose and complement components C3 and C4. Ferritin was detected immunocytochemically only in the macrophages found among the pleural fluid cells. Our study shows that large amounts of ferritin accumulate locally in the pleural cavity in certain types of pleural inflammation. The accumulation is probably partly the result of increased local reticuloendothelial system activity. Determination of the concentration of ferritin in pleural fluid may provide corroborative information for differential diagnosis and may further our understanding of the pathogenetic events that lead to the perpetuation of inflammatory activity in pleural effusions.