Bacterial occurrence in treated root canals, even in patients without post-treatment apical periodontitis, raises the possibility that factors other than mere bacterial presence can be determinants for a favourable outcome of endodontic treatment. Because these factors may be related to the bacterial communities colonizing the root canal, including virulence, density and interactions, the objective of this study was to compare the community structures found in root-canal-treated teeth with (12 samples) and without (11 samples) apical periodontitis lesions by means of a PCR-denaturing gradient gel electrophoresis fingerprinting approach. Results confirmed a polymicrobial composition even in treated patients without post-treatment disease. A large microbial community diversity was observed for treated teeth both with or without disease, but no specific pattern was detected for diseased teeth. Nevertheless, the number of bands from samples with apical periodontitis lesions was statistically significantly higher (P=0.04) than that from samples collected from root-canal-treated teeth without post-treatment apical periodontitis. Furthermore, predominant bands in samples from patients with apical disease were also observed. © 2010 SGM.