This article presented a case of discovering and diagnosing three roots with four canals of the maxillary first premolar. We found and located the extra root canal by clinical diagnosis, careful observation during the operation, and mul-tiangle X-ray. We further confirmed the existence of the three roots with four canals with the help of cone-beam computed tomography. Finally, we verified the success of the high-quality root-canal therapy through root optical microscopy.
