Many different conditions result in the accumulation of pleural fluid. A diagnostic thoracentesis should be performed on all patients with pleural effusion from whom pleural fluid can be easily obtained. Empirically we have found that when the pleural effusion is more than 10 mm thick on the lateral decubitus roentgenogram, pleural fluid is easily obtained. At least 30 cc fluid should be obtained and distributed to the various laboratories as outlined in Table 2. The results of these tests will show whether the fluid is a transudate or an exudate. If the fluid is a transudate, no further diagnostic procedures need be directed towards the pleura. If the fluid is an exudate, the diagnosis will frequently be made by these original tests and therapy for the pleural disease can be instituted. If the diagnosis has not been made, the results of these tests should lead to a rational diagnostic attack.