Presence of a tooth with simultaneous lesions of endodontic and periodontal origin is a challenge to the clinician as far as diagnosis, prognosis and decision-making are concerned. Such infections are typically polimicrobial, and therefore interactions, both antagonistic and synergistic, between different strains and species would be expected. Treatment and prognosis of endodontic-periodontal lesion depend on the cause and the correct diagnosis of each specifying condition. In particular it is critically important to determine whether the lesion is primarily periodontal or primarily endodontic in origin, because the accuracy of diagnosis will determine whether or not the appropriate treatment plan is instigated. Keywords: bacteria, endo-perio lesions, root canal therapy