Radiography by use of the three-dimensional (3D) Accuitomo XYZ Slice View Tomograph (3DX), a device for compact computed tomography in dentistry, was performed as a new diagnostic imaging technique for a patient who needed endodontic therapy. The 3DX was used for examining and diagnosing the presence and expansion of periradicular lesions in each root of a multirooted tooth. High-resolution 3D images were observed and compared with those obtained by routine conventional radiography. It was difficult to diagnose the cause of clinical symptoms by general examinations including the usual intraoral radiography and panoramic radiography. The images obtained by the 3DX, however, clearly showed the apparent presence and expansion of a periradicular lesion in only one root of the multirooted tooth. The 3DX, a newly developed dental computed tomography, provides abundant new image information not attainable by either intraoral radiography or panoramic radiography, and its application for clinical use is very effective for examining and diagnosing regions of interest for endodontic therapy.