Aim: Undesirable complications can occur during endodontic procedures, compromising the success of therapy. This case report examines the management of procedural complications using mineral trioxide aggregate (MTA) in two mandibular molars with failed root canal treatment. Methods: In the first case, a lateral perforation in the mesial root of the mandibular left second molar associated with asymptomatic apical periodontitis was observed. The perforation was repaired with MTA and the mesiobuccal root canal filled with gutta-percha and sealer. In the second case, an apical canal transportation and perforation was identified in the mesial root of the mandibular right second molar associated with apical periodontitis. The root canals were prepared and an apical plug with MTA was made in the mesiobuccal root canal, followed by gutta-percha and sealer obturation. Results: The absence of clinical signs and symptoms and radiographic findings at the 3-year followup indicated a successful treatment of the first case. The tooth of the second case was asymptomatic and functional at the 4-year follow-up, and periapical repair was observed radiographically. Conclusions: The featured cases show that the treatment performed was able to resolve procedural complications during nonsurgical retreatment of morphologically complex teeth. Keywords: apical transportation, mineral trioxide aggregate, nonsurgical endodontic retreatment, root perforation