Mandibular asymmetry is common among orthognathic patients and exhibits great variation. The aim of this study was to propose a new classification of mandibular asymmetry by anatomical regions; namely R (ramus), B (mandibular body) and C (chin), in conjunction with a corresponding 'RBC' three-dimensional (3D) cephalometric analysis. The cone beam computed tomography data of 65 patients with mandibular asymmetry was retrieved to perform the RBC 3D cephalometric analysis and to investigate the characteristics of mandibular asymmetry. It was found that the more posteriorly in mandible, the more pronounced was the vertical asymmetry. Significant transverse asymmetry was only noted in mandibular body. Both mandibular body and chin were significantly asymmetric in length. Seven significant morphologic predictors of menton deviation were identified, namely lower dental midline shift, difference in ramus height, difference in chin length, difference in body length, body height on contralateral side, coronoid height on deviated side and body width on contralateral side, confirming the complex nature of mandibular asymmetry. This simple and concise classification allows comprehensive assessment of mandible morphology by anatomical regions which also facilitates diagnosis, treatment planning and communication in both clinical and research settings.