In this study the incidence and course of pleural effusions (parapneumonic effusions) in patients with acute bacterial pneumonia were prospectively evaluated. Bilateral decubitus chest x-ray films were obtained within 72 hours of admission in 203 patients with an acute febrile illness, purulent sputum and an infiltrate evident on the chest film. Ninety of the 203 patients (44 percent) had pleural effusions. Parapneumonic effusions, which required chest tubes for resolution and/or on which the pleural fluid cultures were positive, were classified as complicated parapneumonic effusions. The 10 patients with complicated parapneumonic effusions had clinical characteristics similar to the remainder of the group and could be separated from the 80 with uncomplicated effusions only by pleural fluid analysis. A pleural fluid pH below 7.00 and/or a glucose level below 40 mg/100 ml are indications for immediate tube thoracostomy. In patients with pleural fluid pH between 7.00 and 7.20 or lactic dehydrogenase (LDH) above 1,000 IU/1,000 ml, tube thoracostomy should be considered, but each case should be individualized; serial studies of the pleural fluid are useful in some of these cases. Patients with pleural fluid pH above 7.20 and pleural fluid LDH below 1,000 mg/100 ml rarely have complicated parapneumonic effusions and do not require serial therapeutic thoracenteses.