Eleven episodes of spontaneous bacterial empyema were identified in eight cirrhotic patients with ascites. Criteria for spontaneous bacterial empyema included positive pleural fluid culture or polymorphonuclear cell concentration greater than 500 cells/mm3, evidence of pleural effusion before an infectious episode and transudate characteristics during infection. In five cases, spontaneous bacterial empyema was culture-negative and was associated with spontaneous bacterial peritonitis. Ascitic fluid was culture-negative in two of these cases and culture-positive in three. Blood cultures were negative in all five of these cases. In six cases spontaneous bacterial empyema was culture-positive (Escherichia coli in four, Klebsiella pneumoniae in one and Clostridium perfringens in one). Four of these patients had the same organism in ascites; one had culture-negative spontaneous bacterial peritonitis and one had no infection of ascites. Blood cultures were positive in four of these patients; three died. Death was more frequent in patients with positive cultures than in those with negative ones (p less than 0.05). Patients with hydrothorax are prone to spontaneous bacterial empyema. This infection probably occurs through hematogenous seeding, but transfer of infected ascites from the abdominal cavity through the diaphragm cannot be excluded. Patients with spontaneous bacterial empyema may be asymptomatic or may be seen with fever, chills and dyspnea. Spontaneous bacterial empyema must be differentiated from parapneumonic empyemas. The presence of pleural effusion before the infectious episode, fluid characteristics and the organisms isolated are the clues for differential diagnosis. Treatment includes antibiotics; chest tube insertion probably is not necessary.