This report describes root canal treatment through cone beam computed tomography and magnification through a surgical microscope of C-shaped mandibular second premolars in the same patient. The mandibular left second premolar with Sert and Bayirli type XV classification was treated for symptomatic irreversible pulpitis and apical periodontitis. The mandibular right second premolar with Vertucci type V classification was re-treated because of a deficient previous root canal treatment, which also involved buccal canal transportation, furcal perforation and two separated instruments in the lingual canal besides symptomatic apical periodontitis. During the root canal re-treatment, buccal canal transportation and furcal perforation were sealed with mineral trioxide aggregate (MTA-Angelus, Londrina, PR, Brazil), separated instruments were removed and a better obturation was achieved. In the 6-month follow-up the patient reported no symptoms, the radiographs of both teeth showed no apical radiolucencies and the percussion tests were negative. For this case, cone beam computed tomography and magnification were essential to identify this unusual root canal morphology, allowing successful endodontic treatment to be achieved. Keywords: C-shaped mandibular premolars, cone beam computed tomography, Sert and Bayirli type XV root canal configuration