Introduction: To evaluate cone-beam computed tomography (CBCT) images in order to determine the presence of mandibular second molars with C-shaped canal system and classify them. Methods: 3035 CBCT images fulfilling the selection criteria were observed. Once established the presence of C-shaped canal system, they were classified according to the anatomic and radiographic classification of Fan et al. Data description was made by frequencies and percentages rates, with a 95% confidence interval (IC95) according to score method. Comparisons were assessed by means of the Chi-square test with a significance level equal to 5%. Results: Of the 225 selected patients, 44 exhibited C-shaped canals (20%; IC95: 15% to 25%). 70% (IC95: 56% to 82%) of patients showed a bilateral C-shaped canal system pattern. Regarding to the axial plane -anatomic classification-, there was a significant association between the root third and the configuration (Chi-square=76.89; p<0.05): at the coronal third prevailed the C1 configuration (47%; IC95: 36% to 58%); at the middle third prevailed the C3d configuration (39%; IC95: 28% to 50%) and at the apical third, the C4 configuration (35%; IC95: 25% to 46%). In 72% (IC95: 61% to 81%) of the cases configuration changes were observed at different levels, while in 28% (IC95: 19% to 39%) the same configuration was observed in the three thirds. This difference was significant (Chi-square=14.52; p<0.05).