This study used multislice computed tomography to evaluate the amount of the remaining cervical dentine thickness in human mandibular first molars after coronal flaring with six different orifice shapers. Sixty mesial roots were divided into six equal groups. Coronal flaring was performed by using X-Gates, Endoflare, Pre-RaCe, Protaper SX, SafeSider Pleezer or NRT instruments. Changes in the cervical dentine thickness of the mesiobuccal and the mesiolingual canals were measured before and after coronal flaring by using multislice computed tomography and image analysis software. Statistical analysis of the mean change percent in the cervical dentine thickness showed that for the mesiobuccal canals, there were no statistically significant differences between the experimental groups and also between the mesial and distal cervical wall thicknesses within the same group (P > 0.05). The same result was obtained after coronal flaring of the mesiolingual canals except in the group prepared with the SafeSider Pleezer instruments, where the change in percent of the distal cervical wall thickness was significantly more than the mesial one (P = 0.05). Keywords: computed tomography, coronal flaring, remaining dentine thickness