Aim: The purpose of this study was to evaluate root and canal morphology of mandibular permanent first molars in a North American population. Methods: A total of 248 cases with bilateral mandibular first molars were evaluated after meeting inclusion and exclusion criteria. All scans from cone beam computed tomography (CBCT) were carefully reviewed by two endodontists. The number of roots, occurrence of a distolingual root, and Vertucci's classification of canal types for each molar were recorded. Differences by gender and ethnicity were calculated using chi-square test and Fisher's exact test. The intra-rater reliability was assessed using Cohen's kappa statistic. Results: The symmetry rate of molars was 85.5%. There was a statistically significant difference in the occurrence of three-rooted mandibular first molars in Asians compared with other ethnic groups (P 0.0001). The most common type of root and canal morphology for mandibular first molars was two separate canals in the mesial root and one canal in the distal root. There was a statistically significant difference in Vertucci's classification of canal types amongst five ethnic groups (African- American, Asian, Caucasian non-Hispanic, Hispanic and others; P 0.0001). Conclusions: If necessary, a CBCT scan, as an adjunct to periapical radiographs, could assist in the identification of root and canal morphology of mandibular permanent first molars to facilitate root canal treatment. Keywords: cone beam computed tomography, mandibular first molar, root canal morphology, tooth anatomy