Summary.The root canal anatomy of 19 mandibular second molars with C‐shaped canals was investigated by rendering the roots transparent and allowing the canal system to be observed by black ink infiltration. The presence of three root canals was most frequent, and lateral canals were found in all roots. Transverse anastomoses were found in 15 of the 19 roots. The apicaf foramen was most commonly situated away from the apex, and apical deltas were found in 16 of the 19 roots. The C‐shaped canals were found more frequently in Asians than in other racial groups(P<0.02).The formation of C‐shaped roots and their racial predilection is discussed. A case report of the root canal treatment of a C‐shaped canal is included and suggestions are made concerning suitable procedures for use in root canal treatment of these complex canal