Introduction: External root resorption occurs as a consequence of trauma to the periodontal attachment apparatus. This damage can be induced by trauma, orthodontic therapy, surgical procedures or periapical inflammation. Healing of the external root resorption can only be achieved by finding and therapy of the particular stimulus. Methods: The present case describes external root resorption of two vital teeth caused by a large lesion of endodontic origin associated with an adjacent non-vital molar. Results: Treatment of the endodontic pathosis resulted in healing of the external root resorption and the lesion of endodontic origin. Conclusions: A lesion of endodontic origin can provoke periapical lesions on adjacent teeth. Standard endodontic diagnostic steps are absolutely essential. Inadequate preliminary diagnosis or prognosis assessment can result in endodontic failure. Keywords: endodontic lesion, external resorption