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Anaerobes, primarily bacteroides and anaerobic streptococci, were detected in 19% of 226 culture-proven empyemas treated since 1950 at five Seattle hospitals. Anaerobic empyemas existing on admission were often chronic and associated with complicated pulmonary infections in older men. However, anaerobes were isolated from empyemas of diverse cause, and no unique association with specific diseases was noted. Most anaerobic empyemas were loculated and difficult to evacuate; 43% of patients required open drainage. Mortality was 22% for aerobic and 19% for anaerobic empyemas. Progressive local infection, chest wall cellulitis, and metastatic abscesses were not observed. Actual incidence of anaerobic empyemas probably exceeds 19%, since culture-positive empyemas represented only 47% of all patients with empyema. Omission of anaerobic cultures would have raised the number of culture-negative empyemas by 14%.