Post treatment periapical periodontitis is usually caused by residual microbes, due to poorly treated root canals and microleakage. Our clinical case proved that orthograde, single-visit endodontic re-treatment is the first choice for the clinician. Cone Beam Computed Tomography (CBCT) images detect the presence and the real extension of the periapical periodontitis and the outcome of the endodontic treatment, in terms of healing the bone defect. Compared with the limited 2-D data obtained by using the radiograph, the CBCT shows a 3-D image of the tooth, the root canal, and the surrounding tissue.