A rare case of a periodontally induced endodontic lesion in a systemically healthy subject of 28 years is described. The patient, having presented with severe attachment loss on the palatal and distal aspects of his maxillary right second premolar, was diagnosed with localized aggressive periodontitis. He had never received periodontal treatment. The tooth was nonvital and showed all signs of symptomatic apical periodontitis. It was also free of any restoration. All clinical findings clearly suggested that the endodontic problem was caused by the aggressive periodontal disease. Bacteriologic screening of the pocket and the root canal, by using "checkerboard" DNA-DNA hybridization analysis, revealed diverse flora in the periodontal lesion. The sample obtained from the root canal exhibited DNA from a limited number of species, including black-pigmented anaerobic rods. No bacterial DNA was found in the root canal that was not also recovered from the periodontal pocket.