To determine the levels of apolipoprotein A1 and apolipoprotein B in pleural effusions and analyze their possible diagnostic value.A total of 117 patients with pleural effusion (30 transudates and 87 exudates) were included. The apolipoproteins were measured by turbidimetry.The apolipoprotein B values in serum were slightly lower in transudates than in exudates, and both apolipoproteins had lower values in patients with benign exudates than in malignant ones. The apolipoprotein levels in pleural fluid were lower in transudates than in exudates. The pleural/serum fluid ratios of both apolipoproteins were significantly lower in malignant effusions compared with benign exudates. No cutoff value was found that would make it possible to differentiate between transudates and exudates or between benign and malignant exudates with sensitivity or specificity levels that had clinical interest.Apolipoprotein A1 and apolipoprotein B levels in pleural fluid are different in transudates and exudates, and the pleural/serum fluid ratios are also different between benign and malignant exudates. However, their measurement does not supply additional clinical information.