Objective:To determine the composition of the microbial flora present in teeth after the failure of root canal therapy

to provide bacteriological basis for curing infected root canals with sensitive drug. Methods:86 root-filled teeth with persistent periapical radiolucencies were selected for retreatment. After removal of the root-filling material

the canals were sampled with paper points. The samples were grown under aerobic and anaerobic conditions and the bacteria were analyzed. Meanwhile

25 teeth with chronic periapical periodontitis without endodontic treatment were selected. Samples were taken and cultivated. Results:Facultative anaerobes was the most prevalent species isolated from canals of root filled teeth. The microbial flora was mainly of 1 to 2 strains of predominantly gram-positive organisms. Enterococcus faecalis was the most commonly recovered bacterial species. Conclusion:Bacteria are high correlation with the teeth with failed endodontic treatment. The composition of the microbial flora in teeth with failed endodontic treatment has its own identity.