Introduction: The purpose of this study was to evaluate the effects of mandibular symphyseal distraction osteogenesis on dental and mandibular skeletal structures with a tooth- and bone- borne distraction device. Methods: The sample comprised 20 patients, ranging in age from 15.8 to 23.3 years (mean, 20.1 +/- 2.3 years) at the start of treatment. The distraction device was activated 1 mm per day. The device was usually maintained in position about 90 days after surgery. Records were obtained at the start of treatment, at the end of distraction (11.4 +/- 2.2 days after surgery), and at follow-up periods (24.1 +/- 4.2 months after surgery). The records included posteroanterior cephalograms and study casts. The data were analyzed statistically by using the repeated measure analysis of variance and paired t test. Results: Posteroanterior cephalometric analysis demonstrated no significant changes in bigonial widths and ramal angles at the end of distraction period. On the other hand, bimolar widths were significantly increased, whereas bicondylar widths were markedly decreased. The dental cast analysis indicated that the maximum amount of increase was found between the mandibular canines, and the widening effect gradually decreased from the mandibular canines to the second molars. The follow-up data confirmed that the treatment results were stable. Conclusions: The long-term findings indicate that mandibular symphyseal distraction osteogenesis is an efficient nonextraction treatment alternative for mandibular dental crowding to increase mandibular skeletal and dental arch widths.