Spontaneous regeneration of bone tissue after mandibular resection is rare in adults, although it does often take place in children. Periosteum conservation appears to play a major role in this healing process. We here report regarding a 5-year-old boy who exhibited a large mandibular trabecular juvenile ossifying fibroma. The lesion was treated by mandibulectomy, with careful preservation of the periosteal layer and immediate reconstruction with a costal graft by an intraoral approach. Monitoring over the course of a year revealed spontaneous mandibular regeneration, and it allowed for a series of measurements of the graft to be made. During this follow-up period, the mandibular height increased from 41.5% to 75.2% (P = 0.0008) of the height of the unaffected mandibular height, while the width grew from 34.4% to 82.8% (P = 0.0078) of the width of the healthy side, thus demonstrating the importance of a conservative approach regarding the periosteum in such situations. The costal graft acted as a support for bone regeneration by immobilizing the remaining bone fragments and by preventing soft-tissue prolapse.