It is now clear that the prognosis in children with histiocytosis X has improved considerable over the past few years. To be sure, patients with solitary lesions have an excellent prognosis. Whereas the outlook for patients with significant visceral involvement is not as good as those with bone lesions only, the outlook is by no means hopeless, as was once thought. A number of prognostic factors have been reviewed here. The most significant of these factors at the present time would appear to be age of onset of the disease, extent of involvement, the rapidity of progression of the disease, and, in particular, the presence or absence of dysfunction of such crucial organ systems as liver, lung, and hemopoietic system. Further studies of the significance of histologic features and immunologic findings are clearly needed to further our understanding of this disorder.