The difference between exudates and transudates is the first question a clinician must solve when facing a pleural effusion. A great number of parameters have been tried without a definite efficacy of any of them. Cholesterol is an easy, useful determination for distinguishing exudates from transudates. In our series of 86 patients a cholesterol value of 50 mg/dl allowed us to correctly classify 94.2% of effusions. The ratio between pleural fluid cholesterol/serum cholesterol was more efficient because it permitted to correctly classify 97.7% of effusions, in one of those major groups which constitute the binomial exudate/transudate.