The mandibular second premolar can be considered one of the most challenging teeth to treat endodontically; due to the complexity of its root canal morphology and increased incidence of multiple canals. The knowledge of internal anatomy of root canals and their possible variations as well as use of magnification, e.g. operating microscope, radiographic examination and illumination, can increase the chances of finding additional canals and contribute to the success of endodontic treatment. The purpose of the current investigation is to report the successful endodontic treatment of a mandibular second premolar with 4 canals; all of them in one single root.