We reviewed the medical records of 293 patients whose pleural fluid data showed a level of > 50% of lymphocytes, after convincingly excluding their tuberculous origin. The study was conducted at a 480-bed teaching hospital in Lleida, Spain, during the last 7 years. The clinical diagnoses of the patients were defined by known predetermined criteria.1Specifically, transudates and exudates were defined by the criteria of Light et al,2 and the categorization of pleural fluid as malignant relied on either a positive result of cytology or biopsy specimen testing or a known cancer without an alternative explanation for the effusion. The final diagnoses of pleural effusions were as follows: malignancy (139 patients); transudates (86 patients); parapneumonic (2 patients); pericardial disease (19 patients); abdominal surgical procedures (10 patients); pulmonary embolism (6 patients); trauma (5 patients); Dressler syndrome (4 patients); connective tissue diseases (3 patients); and postcoronary artery bypass surgery (1 patient). The primary tumors in the malignant group included the following: lung (54 patients); breast (31 patients); lymphoma (13 patients); ovary (12 patients); and miscellaneous (29 patients). In our hospital, ADA activity is determined by Giusti’s colorimetric method, with 40 U/L serving as the cutoff for the identification of tuberculous effusions. Eight patients (2.7%) in our population surpassed this cutoff as follows: non-Hodgkin lymphomas, three patients (ADA levels, 67.5, 46, and 45.9 U/L); acute lymphoid leukemia, one patient (ADA level, 346 U/L); colorectal cancer, one patient (ADA level, 47.1 U/L); small cell lung cancer, one patient (ADA level, 45.9 U/L); and uncomplicated parapneumonic effusions, two patients (ADA levels, 58.9 and 40.5 U/L). If we excluded transudates, the percentage of false-positive elevations was raised insignificantly (3.9%).