The aim of this in vivo study was to assess the incidence of two root canals in the mesial root of the endodontically treated permanent maxillary first molars of a Saudi Arabian sub-population. A clinical study of 352 root-canal treated permanent maxillary first molars was conducted. The teeth were examined clinically and radiographically. The results showed that 23.3 % of the examined teeth had two canals in the mesial root (17.0 % with one apical foramen and 6.3 % with two separate foramens). The occurrence of two canals in the mesial root of the maxillary first molar of a Saudi Arabian sub-population was within the normal range.King Saud Universit