The efficacy of distraction osteogenesis in an irradiated area is controversial, although this procedure is now widely used in the field of craniomaxillofacial surgery. We report the clinical results from 4 patients with mandibular defects treated by lengthening of the irradiated mandibles. All patients had a mandibular defect caused by ablation of a malignant tumor. They had undergone radiotherapy at a total dose of 30 to 50 Gy to the surgical site after tumorectomy. Distraction osteogenesis was used as the secondary reconstruction method in 6 sites of the remaining irradiated mandibles and in 1 site of the transferred vascularized scapula after radiotherapy. The transported segment was obtained by corticotomy with an initial gap of 0 to 2 mm, and internal extension plates were used. Distraction was commenced after a latency period of 7 to 10 days and performed at the rate of 0.25 to 1.0 mm/d. The total amount of distraction and consolidation periods ranged from 15 to 25 mm and 120 to 193 days, respectively. In 5 of the 6 sites in the remaining irradiated mandibles, satisfactory bone formation in the distraction gap was observed, although a fracture after new bone formation was observed in 1 site. Fibrous callus formation was observed in 1 irradiated site only, and satisfactory results were obtained in another site of transferred vascularized scapula in the same patient. From these experiences, we believe that distraction may provide a reconstruction option for mandibular defects even under irradiated conditions because the procedure is simple and less invasive.