Abstract The purpose of this prospective observational study was to evaluate whether cone beam CT (CBCT) is a useful tool for analyzing the fracture line in a bilateral sagittal split osteotomy (BSSO). The patient group consisted of 40 consecutive patients (9 males and 31 females) with a mandibular hypoplasia who underwent a BSSO advancement (Hunsuck modification; n = 80 splits) between September 2006 and July 2008. The mean age at the time of surgery was 34 years (range 17–61 years). A newly developed lingual split scale was used to categorize the path of the fracture line on the lingual side of the ramus based on one-day postoperative data sets reconstructed from CBCT data. Although all splits (n = 80) were performed according to the standardized protocol, only 51% of the fracture lines run according to the Hunsuck's description, whereas 33% ran through the mandibular canal and 16% split otherwise. The split pattern was influenced by the length of the medial osteotomy (p = 0.01). In conclusion, 3D imaging is a useful tool for analyzing the surgical outcome of a BSSO and has the potential to provide substantial data on the position of the proximal segments as a result of the lingual fracture line.