OBJECTIVE Carcinoembryonic antigen (CEA) is the most frequently used tumor marker in pleural fluid. Nevertheless, little is known about the causes of false-positive results. The aim of the study was to analyze the frequency, etiologies, and characteristics of the nonmalignant pleural effusions associated with elevated levels of CEA in pleural fluid. METHODS Two hundred seventy-three consecutive patients with pleural effusions were evaluated, 91 (33%) associated with malignancy, and 182 (67%) due to benign diseases (51 transudates, 38 tuberculosis, 37 parapneumonic, 56 other). RESULTS A level of CEA in pleural fluid above 10 ng/mL was found in 47% of pleural effusions associated with malignancy. Elevated levels of CEA were also found in 17 of the 182 (9%) nonmalignant pleural effusions: all five empyemas, one of the 23 typical parapneumonic (4%), two of the six borderline complicated (33%), and four of the eight complicated parapneumonic effusions (50%), one of the 38 tuberculous pleurisy (3%), one of the 11 hepatic transudates (9%), in the only patient with urinothorax, in the only patient with acute pancreatitis, and in one patient with postsurgery pleural effusion but with esophageal carcinoma and elevated CEA level in serum. CONCLUSIONS Although an elevated level of CEA in pleural fluid is suggestive of malignancy, CEA can be elevated in 9% of pleurisy owing to benign diseases, especially in empyemas and in complicated parapneumonic effusions. Identifying the most frequent causes of false-positive results of CEA helps to correctly interpret the findings of this tumor marker.