The purpose of this paper was to present the unique case of complete pre-eruptive idiopathic crown resorption, few of which have ever been reported, because most have concerned partial idiopathic crowns. Crown resorption affects only a part of the crown and begins when crown mineralization is complete, leading to hypomineralization of the teeth. Treatment usually consists of restorative/endodontic care or extraction. A 12-year-old-boy presented with complete idiopathic resorption of the permanent mandibular left third molar and secondary failure of eruption of the permanent mandibular left first molar. The finding was incidental during routine radiographs for orthodontic treatment and was documented using cone beam computed tomography and computed tomography (one-mm slices). Follow-up, using a panoramic radiograph, occurred after one year, confirming the absence of the third molar. No preventive or therapeutic regimen is known.