A prospective study of bacteriology in 14 cases of cavitating pulmonary infections and empyema is reported. Bacteriologic results were based on transtracheal aspirates or pleural fluid. Appropriate anaerobic bacteriologic methods were employed. Anaerobic bacteria were recovered in 11 patients (79%); they were the only pathogens isolated in 6 patients. The predominant species were Fusobacterium nucleatum, Bacteroides melaninogenicus, Bacteroides fragilis and Peptostreptococcus. Aerobic bacteria were present in 7 patients. The results indicate that anaerobes play a key role in most cases of cavitating pulmonary infections and empyema, and that proper culture of adequat specimens will generally establish the bacteriological diagnosis.