The work was undertaken owing to great difficulties in diagnosis and treatment of histiocytosis X (HX) in children, which occurs comparatively rarely and its clinical symptoms are of great variety. The analysed material included 45 cases of children with HX treated in one clinical department. A big number of analysed cases makes safe conclusions possible. The aim of this work was evaluation of therapeutic methods used in HX, on the basis of the results obtained from the HX treatment and with consideration of such prognostic factors as: the age of the child when the first pathological symptoms occurred , the extent of the pathological process and the localization of the lesions. The results of the research carried out on 23 children in whom the pathologic process concerned the skeletal system only were evaluated separately. Remaining 22 cases in which the lesions were noticeable not only in the skeletal system, but also in other organs and systems were divided in two groups: group 1 (11 children) included all the cases treated till 1969 by different methods with the use of different cytostatics; group 2 (11 children) included the cases observed by the author introducing a new method of treatment based on intensive therapy followed by the usage of Vinblastine or both Vinblastine and Prednisone. In the evaluation of the extent of the pathologic process and localization of lesions the following symptoms were taken into consideration: skeletal and skin lesions, enlargement of lymphatic nodes and liver, lesions of gums--i.e. movable teeth, lesions in ears, lungs, hematopoietic system, and fever. The presence of these symptoms is proved by the clinical, radiological and laboratory examinations. The age of the patient with the first symptoms of the disease was established through inquiry, the appearance at least one of the HX symptoms such as: protrusion of the soft tissue, skin lesions and discharge from the ears was considered as the beginning of the disease. In the group of children with lesions limited only to skeletal system, the age of the appearance of the first symptoms was between 1-12 years. The greatest morbidity was between the 4-th and the 8-th year of life (Fig. 1). The main methods of treatment were surgery and/or radiotherapy. Chemotherapy was used sporadically as supplementary treatment. In all cases response was good. (Tab. 1.). The generalized form of the disease is observed as a rule among the youngest children. In our own material of 20 cases out of all 22, the first symptoms of the disease appeared before 3-rd year of life was over. (Fig. 4.).(ABSTRACT TRUNCATED AT 400 WORDS)