Adenosine deaminase (ADA) activity was studied in 221 patients with pleuroperitoneal effusions. Patients were subdivided into the following six groups: (1) 48 cases of tuberculosis; (2) 46 with malignancies; (3) 30 postpneumonic effusions; (4) 19 cases of several diseases; (5) 18 patients with pleural effusions of unknown origin; and (6) 60 with acellular transudates. Mean ADA activity was 92.43 +/- 29.43 U/L 37 degrees C in group 1; 13.43 +/- 10.69 in group 2; 19.91 +/- 19.64 in group 3; 14.27 +/- 17.47 in group 4; 14.48 +/- 13.92 in group 5; and 2.29 +/- 3.4 in group 6. Comparing the level achieved in group 1 with all others, the difference is significant at the p less than 0.001 level. Specificity (0.97) and sensitivity (1) of the test in tuberculosis is very high, when a value of more than 45 U/L is considered. In patients with pleural tuberculosis, T-lymphocytes predominate in the fluid but their number did not correlate with ADA-activity (p greater than 0.10). Assessment of ADA in pathologic fluids is of great value in the diagnosis of tuberculosis of the pleura.