A case is presented in which a previous partial root canal treatment on a mandibular molar developed internal resorption of the distal canal. The case was followed at the University of Oklahoma student clinic and endodontic treatment completed in the mesial canals, with calcium hydroxide placed in the distal canal for 10 months due to a pre-existing perforating defect of the root surface. Endodontics was then completed. A 17-month follow-up film showed osseous repair apically and also adjacent to the distal root surface where a slight overextension of filling material was evident.