Eosinophilic granuloma (EG) is a cryptogenic disease associated with infiltration of eosinophilic leukocytes (ELs), hyperplasia of histiocytes, and formation of granuloma. We report a case of EG arising in the mandible that spontaneously disappeared after biopsy. The patient was an 11-year-old boy who had pain, trismus, and swelling of the mandible. Computed tomographic examination showed irregular resorption of bone on the left side of the mandible. Histological examination of a biopsy specimen revealed that the lesion consisted of many-ELs and histiocytes. The pathological diagnosis was eosinophilic granuloma. We performed extirpation under general anesthesia 2 months after biopsy. The second histological examination of a surgical specimen revealed granuloma tissue and giant cells. ELs and histiocytes had disappeared. The pathological diagnosis was granuloma.