Nearly 50 per cent of patients with acute bacterial pneumonia have an accompanying pleural effusion (parapneumonic effusion). With appropriate antibiotic therapy, the pleural effusion will resolve along with the pneumonia in the majority of patients. However, in a small fraction, the pleural effusion will not resolve unless drainage of the pleural space is instituted. Such patients are said to have complicated parapneumonic effusions. It is important to identify patients with complicated parapneumonic effusions as early as possible, since tube drainage of the pleural space becomes increasingly difficult the longer its institution is delayed. The possibility of a complicated parapneumonic effusion should be considered in every patient with bacterial pneumonia. If both diaphragms cannot be distinctly identified throughout their length on the lateral chest radiograph, decubitus chest radiographs should be obtained. If the thickness of the fluid on the decubitus radiograph is greater than 10 mm, a diagnostic thoracentesis should be performed. Only pleural fluid analysis can identify patients with complicated parapneumonic effusions. Complicated parapneumonic effusions are characterized by low pleural fluid pH and glucose levels, a high pleural fluid LDH, and a positive Gram stain of the pleural fluid. Tube thoracostomy should be performed immediately in a patient with an acute bacterial pneumonia if the pleural fluid glucose is below 40 mg per 100 ml, the pleural fluid pH is below 7.00, or if the Gram stain of the pleural fluid is positive. Patients with pleural fluid pH above 7.20, pleural fluid LDH below 1000 IU per L, and pleural fluid glucose levels above 40 mg per 100 ml respond well to only the administration of appropriate antibiotics.(ABSTRACT TRUNCATED AT 250 WORDS)