The large bilateral mandibular retromolar canals were found in a 47-year-old Japanese man. This paper presents the nature and distribution of the elements of nerves and arteries in the left retromolar canal which were able to examined. The left mandibular retromolar canal arose from the bifurcation of the mandibular canal at 18 mm anterior to the mandibular foramen and it opened on the surface of the retromolar fossa at 13 mm posterior to the third mandibular molar. The total length of the retromolar canal from the bifurcation to the retromolar foramen was about 16 mm, and 2.5-2.6 mm in diameter. The radiographic image of this retromolar canal was a liner shadow with thin radiopaque anterior and posterior borders cast on a lateral radiography of the left half of the head. Within the retromolar canal, the artery that was branched from the inferior alveolar artery ran through the canal forwards and joined with the branches of the buccal artery and the facial artery, and then gave off the superior and inferior labial arteries. The nerve (R. retromolaris) in the retromolar canal derived from the inferior alveolar nerve trunk and branched off to the following areas: the third mandibular molar, the mucosa of retromolar triangle, the buccal mucosa, and the buccal gingiva of the mandibular premolar and molar region. The retromolar canals and foramina were found in 8 out of 41 Japanese skulls. Clinically, if the arteries in the retromolar canal become injured, complications may arise such as bleeding, and iatrogenic nerve damage may lead to paralysis.(ABSTRACT TRUNCATED AT 250 WORDS)