Letterer-Siwe disease, Hand-Schuller-Christian disease and eosinophilic granuloma of the bone are merely different clinical expressions of one disease. Lichtenstein grouped these three clinical syndromes and proposed that this disease be termed histiocytosis X. This grouping was based on the histologic similarities and the possibility of transformation of one syndrome into another. The cause of histiocytosis X remains unknown. Histiocytosis X is a condition that presents single or multiple lesions, distributed in the soft tissue, lymph nodes, various organ and bones, especially in the areas of marked reticuloendothelial activity. The authors reviewed the clinical findings, radiographs and the treatment of the twenty-six patients with histiocytosis X diagnosed on the base of pathologic findings at Severance Hospital, between January, 1971 and December, 1980. Among the twenty-six patients, twenty-one patients could be followed, ranging from one month to six years, with an average follow-up of 1.6 years. The results obtained were as follows: 1. There was a slight male predominance (61.6%). The age ranged from 2 months to 42 years (average 8.2 years). Sixty-five percents of patients were the child under 4 years of age. 2. Hand-Schuller-Christian disease (50%) was the most common form of this disease. Letterer-Siwe disease developed in the youngest (average 1.2 years) and eosinophilic granuloma in the eldest (average 20.4 years). 3. The common manifestations were hepatomegaly, skin rashes and anemia in Letterer-Siwe disease; palpable mass, pain and exophthalmos in Hand-Schuller-Christian disease; pain and mass in eosinophilic granuloma. 4. The common sites of the skeletal lesions were skull, spine and femur, etc. 5. The patients with single skeletal lesion improved regardless of the methods of treatment. 6. Vinblastine, methotrexate and/or prednisone were valuable agents for the multiply involved patients. 7. Prognosis was poor for the patients who were young at onest, or who had multiple systemic involvements, but in case only with skeletal involvement the prognosis was good. Eosinophilic granuloma is the most benign variant, while Letterer-Siwe disease is the most malignant type.