BACKGROUND
The possibility of mandibular bad spilt might happen during bilateral sagittal split osteotomy (BSSO). This study investigated the effect of impacted mandibular third molars on bad spilt incidence during BSSO.
METHODS 
Totally, 140 patients under 40 years old who were candidates for BSSO surgery due to class 3 skeletal discrepancy were divided randomly into two equal groups. The impacted mandibular third molars were presented in one group during BSSO (Exposed), and the third molars were removed at least six months before surgery for the other group (Unexposed). All cases underwent BSSO using the same technique by a single surgeon. A bad split was diagnosed by inter-operative clinical examination and postoperative panoramic radiography. 
RESULTS
Four bad split occurrences were observed including three patients in the group which impacted mandibular third molars were presented and one patient in the group without impacted mandibular third molars. The incidence of bad fracture in the exposed group was 3.7 times more than the unexposed group. The incidence of the bad fracture in exposed group was 3.7 times more than unexposed group. The chance of fractures in females was 1.7 times higher than males. With one year addition to the patient’s age, chance of fracture increased 0.985 times more. 
CONCLUSION
Overall incidence of bad split fracture in presence of mandibular third molars in females and at older ages increased during BSSO. The extraction of impacted mandibular third molars, six months before the BSSO is recommended to prevent the bad split incidence during the operation.