The bacterial findings of 73 maxillary sinuses in 48 patients with chronic maxillary sinusitis, together with 7 non-inflamed sinuses, are reported. Employing an intraoperative technique and simultaneous collection of 3 different types of specimens in the present investigation made possible comparison of their bacterial characteristics. The results indicated that intraoperative culture of antral mucosa seems to provide the most reliable finding of bacterial flora in chronic maxillary sinusitis. The anaerobic bacteria were never found in the mucosal culture of non-inflamed sinus. The anaerobes appeared to invade the sinus cavity following the sealing of the ostium through the lymphatic or venous system and maintain the inflammatory process. Microbiological analysis of the results between the infected and non-inflamed sinuses established anaerobic bacteria as the most important pathogen in chronic maxillary sinusitis. The predominant anaerobes recovered in descending order of frequency were Veillonella sp., Peptococcus sp., Propionibacterium acne and anaerobic nonspore-forming GPB. Statistical analysis of the results of mucosal culture of inflamed and control materials demonstrated that those aerobic and faculatative bacteria recovered in the inflamed sinus appeared to be the normal inhabitants of non-inflamed sinus mucosa. The presence of normal flora in the normal healthy sinus mucosa may explain the chain of events that follows the occlusion of the ostium. These aerobic bacteria may become pathogenic and play a role in the pathogenesis of sinusitis.