The purpose of this study was to evaluate microscopically the newly formed hard tissue after a consolidation period of mandibular symphyseal distraction osteogenesis (MSDO). Sixteen patients underwent MSDO treatment. After a latency period of seven days, the distraction device was activated by the patient once in the morning and once in the evening, for a total of one mm per day for a mean 10.1 +/- 2.8 days, and the mean opening of the device was 8.1 +/- 1.7 mm. The device was usually maintained in position approximately 90 days after surgery. After the completion of the distraction period, the lower anterior teeth were bonded and tooth movement into the distraction site was initiated. After a consolidation period, second surgery was performed to remove the distraction devices. During the second surgery, hard tissue biopsies were taken on the apical region of the two central incisors and the left canine. The samples were fixed in 10% buffered formalin and decalcified in 3% HNO3 solutions. New bone formation was present within the distraction gap immediately after the consolidation period. The cellular construction was more irregular in the distraction sections than in the normal bone sections. The newly distracted area was not complete immediately after the consolidation period. Furthermore, the newly formed bone had a membranous structure, which indicates continual maturation. Bone exposed to stretching forces undergoes new bone formation, and the newly formed bone is of a membranous type also named as a woven type.