One hundred eighty-three patients had simultaneous blood and pleural fluid pH determinations. Thirty-six effusions were transudates, and 147 were exudates. In 46 effusions, the pleural fluid pH was less than 7.30; all 46 were exudates. A pleural fluid pH less than 7.30 was associated with the following six diagnoses: (1) empyema; (2) malignancy; (3) collagen vascular disease; (4) tuberculosis; (5) esophageal rupture; and (6) hemothorax. The results of pleural fluid pH determination are immediately available, narrow the differential diagnosis of the exudate, and may expedite patient management. The pH of pleural fluid should be measured whenever a diagnostic thoracocentesis is performed.