Background: Eosinophilic granuloma (EG) is the mildest and mainly localized form of the clinicopathologic spectrum of Langerhans' cell histiocytosis. It is a destructive osseous lesion characterized by a vast number of eosinophils and histiocytes. The etiology remains unknown. In this paper, a case of EG is presented that was initially diagnosed and treated as aggressive periodontitis (AP).Methods: Despite treatment procedures, the EG continued to expand very quickly, destroying the lingual cortical bone and the neighboring soft tissues and exhibiting periosteal reaction. Diagnosis of EG was established on the basis of histopathologic and immunohistochemical evaluation. Moreover, certain manifestations in the skeletal and respiratory system were observed.Results: Surgical curettage of the lesions was effective; however, corticosteroids and low‐dose radiation were used as adjunctive therapy.Conclusion: The rapid progress of eosinophilic granuloma, the diagnostic problems, and the consequences of late diagnosis and treatment are discussed.