Langerhans'-cell histiocytosis （LCH） is caused by monoclonal proliferation of Langerhans' cells, a type of antigen-presenting cell, and affects multiple organs. We report a case of LCH arising in the mandible of a young boy.<br>The patient was an 11-year-old boy who referred to our hospital because of swelling of the right cheek. Computed tomographic examination showed a destructive bony lesion extending from the right mandibular body to the mandibular angle, with resorption of the buccal and lingual cortical plate. Gadolinium-enhanced T1-weighted magnetic resonance imaging showed contrast enhancement in a large portion of the right mandibular body, with soft tissue extension. A biopsy of the lesion was performed with the patient under general anesthesia because the radiographic findings suggested malignancy. Histopathological examination revealed that the lesion was composed of eosinophilic cells and histiocytes, which were positive for S-100 and CD1a immunohistochemically. These findings led to the diagnosis of LCH. PET-CT showed no evidence of lesions in other organs or bone.<br>Patients with unifocal LCH arising in one organ generally have a high chance of spontaneous remission and favorable outcomes. The present case spontaneously remitted 3 months after the biopsy, without additional treatment. Currently, 1 year 9 months after the biopsy, the patient has made favorable progress, with no recurrence.