Cholesterol and carcinoembryonic antigen (CEA) levels in pleural effusion and sera, were measured in 199 patients with pleural effusions of various origins. Malignant cause was found in 93, and nonmalignant in 106 patients. Mean cholesterol level in sera of patient with malignant disease was 5.0 +/- 0.93 mmol/L, and in nonmalignant group 4.34 +/- 1.32 mmol/L. The difference was not statistically significant. Mean cholesterol level in nonmalignant pleural effusions was higher thAn those in malignant (2.51 +/- 1.23 mmol/L; and 2.28 +/- 1.06 mmol/L), but the difference was also not significant. Average pleural fluid/serum cholesterol ratio (Holl/S) in nonmalignant group was 0.61 +/- 0.32 and in malignant group 0.46 +/- 0.22. The difference between those mean values was significant. Higher ratio, at the cut off value of 0.5 was found in 79/106 and in 25/93 malignant patients. Calculated sensitivity was 75%, specificity 73%, positive predictive value 76%, negative predictive value 65% and accuracy 69%. Significant negative correlation between Holi/S and pleural fluid CEA was found (p < 0.05). It was assumed that pleural fluid/serum cholesterol ratio lower than 0.5 could be of great benefit, as an additional test in the differentiation of malignant from benign pleural effusion.