The high morphological complexity of premolars, has led to propose different techniques for successful endodontic therapy. The purpose of this report is to describe the ProTaper manual technique in a second mandibular premolar showing a high anatomical complexity. A patient of 19 years old co- mes to the clinic of endodontic in the Regional Hospi- tal (Temuco), diagnosed with irreversible pulpitis on tooth 35. Radiographic examination found that the tooth has two roots. For clinical suspicion of the presence of a third canal a Cone Beam computed tomography was requested, which confirmed the existence of a distolingual canal. Because it is a complex case, the increased taper endodontic technique was performed, allowing us to complete the endodontic therapy in less time and with less difficulty.