The root canal treatment of mandibular premolars is challenging due to their aberrant root canal anatomy and morphology. These teeth usually have a single root and a single canal, but anatomic variations can occur. The successful non-surgical root canal treatment of such teeth requires the basic knowledge of root canal anatomy and variation from the norm. The presence of an extra root or the extra canal is difficult to detect in periapical radiographs and overlooked root canals may lead to failure of the root canal treatment. This case report describes the use of spiral computed tomography (SCT) in confirmatory diagnosis and the root canal treatment of a mandibular second premolar with three roots and four canals. Schlagwörter: anatomic variations, mandibular premolar, spiral computed tomography