Aspirate of pus from 5 periapical abscesses of the upper jaw and their corresponding maxillary sinusitis were studied for aerobic and anaerobic bacteria. Polymicrobial flora were found in all instances, where the number of isolates varied from 2 to 5. Anaerobes were recovered from all specimens. The predominant isolates were Prevotella sp., Porphyromonas sp., Fusobacterium nucleatum, and Peptostreptococcus sp. Concordance in the microbiological findings between periapical abscess and the maxillary sinus flora was found in all instances. However, certain organisms were only present at one site and not the other. Five beta-lactamase-producing organisms were present in the five specimens. These data confirm the importance of anaerobic bacteria in periapical abscesses and demonstrate their predominance in maxillary sinusitis that is associated with them.