Swabs of 126 chronically inflamed maxillary sinuses were processed for aerobic and anaerobic bacteria during endoscopy. Bacterial growth was present in 113 of 126 specimens (90%). Anaerobic bacteria were isolated in 100 of 113 culture-positive specimens (88%). Anaerobes only were recovered in 59 cases (52%) and, in 41 (36%), they were mixed with aerobic or facultative bacteria. Aerobic or facultative bacteria were present in 13 cases (12%). A total of 323 isolates (2.8 per specimen), 228 (2.3 per specimen) anaerobes and 95 (1.7 per specimen) aerobes or facultatives, were isolated. The predominant anaerobic organisms were anaerobic cocci and Bacteroides species; the predominant aerobes or facultative bacteria were Streptococcus species and Staphylococcus aureus. These findings indicate the major role of anaerobic organisms in chronic sinusitis.