The aim was to report the diagnosis and successful clinical management of a patient with pathological anatomic variation in a mandibular first molar, caused by internal and external resorption and to evaluate radiographically the long term clinical outcome of the root canal treatment. A case is presented in which a previous caries removal procedure with pulp exposure on a mandibular molar developed into internal resorption of the pulp chamber floor and the distal canal. A preoperative periapical radiograph showed apical external resorption of the molar roots and condensing osteitis associated with the mesial root. Root canal treatment was completed in two visits, with calcium hydroxide used as an interappointment dressing for 1 month. A 36 month followup showed apical root repair and no evidence of periapical bony changes. Keywords: condensing osteitis, external resorption, internal resorption