A cone-beam CT (CB-CT) that can make three-directional images with high resolution using a flat-panel detector has been applied todiagnoses in the oral and maxillofacial region, such as the assessments of impacted tooth position, morphology of the temporomandibular joint and dental implant placement. Several investigators have also reported the usefulness of CB-CT during surgical useful dentistry. CB-CT can provide the valuable information for properly determining the treatment plan for endodontic-periodontal lesions. We report a case of endodontic-periodontal lesion that could not be indentified by conventional radiographic examinations but was appropriately treated based on CB-CT finding so.