The authors have recently experienced a case of Histiocytosis-X which's followed up for one year. A 4 years old female was admitted to our O.R.L. department because of swelling of left cheek, purulent otorrhea and generalized weakness with pallor, so that blood transfusion was done to control the anemia, and then biopsy was performed from the mass on the left maxillary bone which's replaced a thick yellow fibrous capsule in which there's filled with blood clot (dark brown color). The microscopic finding revealed marked histiocytic proliferation. On the physical examination, hard mass was palpated on the left cheek, left eye ball was slightly prominent and the left external audiotory canal was shown swelling with a little erosion of the posterior wall and purulent otorrhea from the left. The left ear drum was not seen due to swelling at the external auditory canal. There's splenomegaly with hepatomegaly too. The X-ray films were revealed that paranasal sinuses were generally hazy and nasal cavity was bilaterally cloudy, more markedly in the left. Bony irregularity in association with erosion and or destruction involving inferior margin of left orbit and upper border of the left maxillary antrum made out. Right mastoid was hazy and air cells were diploic in appearance. Skull (AP & Lat.) revealed no evidence of abnormality. Long bone series revealed within normal limits. The hematology with other laboratory tests including bone marrow study and Hickey Hare test were in within normal limits except mild anemia. Her conditions including swelling of external auditory canal, purulent otorrhea from the left and swelling of the left cheek were initially controlled by the therapy of steroid and antibiotics. About 7 months later, irradiation therapy (1,750 r) was performed on the both maxillary bones. At that time the X-ray of skull revealed that multiple ""punched out"" lesions and suture separation were developed. After the irradiation, the suture separation was returned almost to normal limit and improving the skull lesions too. She was unfortunately carried again to us complaining with protruding of the right eye ball and right cheek as like initial signs of the left face about 10 months later from the first admission. Therefore irradiation therapy(1,000r) was started on the right maxilla with steroid therapy and then the local signs improved somewhat. Her local lesions on the left maxilla firstly was eosinophilic granuloma by the biopsy but the lesions lastly involved to the skull and right maxillary bone despite of irradiations and steroid therapy. Briefly, the authors were thought that the disease's course was initially developed from eosinophilic granuloma and eventually changed to Hand-Schüller-Christian disease. Many authors proposed that the entity of Histiocytosis-X was together with Eosinophilic granuloma, Hand-Schüller-Christians's disease and Letterer-Siwe disease. According to our experience, authors consented that the name of the mentioned three diseases belongs to a category of Histiocytosis-X.