This report describes a rare palato-radicular groove in a maxillary lateral incisor with an accessory root and an atypical root canal. Failure to identify the presence of the palatal groove allowed infection to persist, which led to palatal swelling and labial fistula. During endodontic retreatment of the infected root canal, an accessory root with a second root canal was detected. Owing to the extent of the circumferential bone loss and the atypical root canal, the second root was resected. Histological analysis of the resected root suggested there was reparative dentine in the deep groove and several accessory canals. The structure of the reparative dentine was irregular. In the apical third of the resected root, the canal was oval with a wide slit-type aperture. It was possible that the combination of the deep groove and the accessory canals with the atypical communication to the periodontal ligament was the reason for retrograde pulp inflammation and pulp necrosis. Rapid healing after surgical treatment was observed. At the 6-month follow-up examination, the patient was symptom-free. No swelling or exudation was present. Radiographic investigation of the lateral bone defect showed the beginning of ossification and healing. Schlagwörter: endodontic-periodontal lesion, ledges, palato-radicular groove, radicular palatal groove