Eosinophilic granuloma (EG) is the mildest and localized form of a group of diseases named Histiocytosis X. It is a destructive osseous lesion characterized by presence of a vast number of eosinophils and histiocytes .It has a neoplastic nature especially in the chronic forms. Based on the site of the lesion, three types are elucidated: 1- intraosseous 2- alveolar 3- mixed. In the last two types, extensive alveolar involvement and loosening of the teeth clinically may resemble aggressive periodontitis (AP). We report a case of EG which was initially diagnosed and treated as AP. The rapid progress, diagnostic problems, etiologic factors and the consequences of late diagnosis and treatment of eosinophilic granuloma are discussed. This explicates why dentists need to know the differential diagnosis of EG with AP for early diagnosis and treatment.