We found a dens invaginatius radiographically derived in a ten-year, ten-month old girl who was referred to the pediatric dental clinc, Nihon University School of Dentistry at Matsudo with spontaneous pain of the maxillary left lateral incisor, and we surmisd that the dens invaginatus induced periapical periodontitis.<BR>The patient had a fever of 38.0°C, diffuse swelling around the left nosewing and pulsating pain. The number of leukocytes was 1.2×10³/μl at the time of her first medical examination. The radiographic examination showed a radiopaque invaginations extending from the cingulum to the root in the affected tooth, and a periapical radiolucent lesion. Based on the above examinations, we diagnosed her having periapical periodontitis with dens invaginatus of the maxillary left lateral incisor. We conducted an examination of the tooth using 3 DX® (Morita Co. ) in order to confirm the complicated morphology of the dens invaginatus before dental treatment. As a result of images from 3 DX®, we found that the part of invagination was extended from the top of the tooth crown to a distance,19mm into the root (tooth size was 21 mm) in the maxillary left lateral incisor. The direction of the invagination turned toward the mesial site and the apical of invagination did not correspond to the root apex of the lateral incisor. We found two holes in the root canal of the teeth when endodontic treatment was performed with the information from the 3 DX®. One root canal showed a vital response with breeding and pain, and the necrosis contents were observed from another root canal. The root canal treatment and root canal filling with the gutta-percha point therefore were performed on the necrotized root canal. The patient had no symptoms after treatment, and the maxillary left lateral incisor still had a positive response to sensitivity testing.<BR>The 3 DX® images were useful for the diagnosis and treatment of dens invaginatus with complicated morphology.