Pleural effusion may complicate various diseases. To facilitate the differential diagnosis of pleural effusion, the authors conducted an epidemiologic study of incidence and etiology of different types of pleural effusions in a well-defined region in central Bohemia. During a 1-year period, the authors worked with the general practitioners and other physicians from the local hospital to identify patients with pleural effusion either while the patient was alive or at autopsy. During this period, 142 individuals (93 living and 49 at autopsy) with pleural effusion were identified among a population of 44,000 (incidence = 0.32 percent). The most common etiologies of the effusions were congestive heart failure (65, 46 percent), malignant effusions (31, 22 percent), parapneumonic effusions (24, 17 percent), and pulmonary emboli (8, 5.6 percent). Other etiologies for the effusions included pulmonary hemothorax (6), intra-abdominal processes (4), uremia (2), myxedema (1), and rheumatoid pleuritis (1). We conclude that pleural effusions are relatively common and if these figures are extrapolated to the United States and Czechoslovakia respectively, one could expect at least 800,000 cases in the United States and 48,000 in Czechoslovakia annually. Over 90 percent of the pleural effusions will be due to congestive heart failure, malignancy, pneumonia, or pulmonary emboli.