It has been stated that pleural fluid eosinophilia (defined as greater than 10 percent eosinophils in the pleural white cell differential count) is not helpful in the diagnosis of exudative effusions. By review of the recent literature, it was found that pleural fluid eosinophilia was associated most often with idiopathic effusions or with air previously introduced into the pleural space. Also, a high proportion of "idiopathic" and benign asbestos effusions were characterized by pleural fluid eosinophilia, a previously unrecognized phenomenon. The diagnostic utility of finding eosinophils in the pleural space was assessed from its impact on prior probabilities of disease. Estimates of pretest likelihoods of malignant versus nonmalignant pleural effusions and the prevalence of eosinophilia in effusions of known cause were obtained from extensive literature review. These were modified by using Bayes' rule to estimate the revised probability of disease in the presence of an eosinophilic effusion. The presence of pleural fluid eosinophilia considerably reduced the probability of malignancy or tuberculosis and increased the likelihood of an underlying benign disorder. Pleural fluid eosinophilia is a useful finding that can aid in the diagnosis of an exudative pleural effusion.