Total and differential leukocyte counts in pleural fluid were determined prospectively in 140 consecutive patients undergoing diagnostic thoracentesis. The total number of leukocytes was higher in exudates than in transudates. Lymphocytes predominated overwhelmingly (more than 80% of all leukocytes) in 29 of the 31 effusions with verified or probable tuberculous etiology and in 18 of the 24 with malignant etiology, as well as in 20 exudative pleural effusions of other causes. The significant differences in the percentage of lymphocyte distinguished tuberculous effusions from parapneumonic (p less than 0.001), nonspecific (p less than 0.001) and rheumatoid effusions (p less than 0.005), as well as from effusions caused by undefined connective tissue disease (p less than 0.001). Eosinophilia (more than 10%) of the pleural fluid was not present in any patient with proven tuberculosis and in only one patient with malignant effusion (malignant mesothelioma) but did occur in 12 of the 26 patients with nonspecific effusion. These results show that a great lymphocytic predominance is characteristic particularly of tuberculous but also of malignant pleural effusions, although this predominance cannot be regarded as disease-specific. Eosinophilia of the pleural fluid--usually a nonspecific sign--indicates nontuberculous, nonmalignant etiology.