Eosinophilic granuloma may be defined briefly as a chronic inflammatory disorder of unknown etiology characterized primarily by the presence of histiocytes and eosinophils. General awareness of this disease may be said to be a consequence of two almost simultaneous but independent studies which appeared in 1940,1,2 although Mazzitello3 mentions the existence of less well-appreciated earlier descriptions. After 1940 a series of papers appeared in considerable number, and a clinical and pathological spectrum has gradually evolved.
Histologically, many of the cases have shown apparent transitions with two other disorders, namely, the Hand-Schüller-Christian syndrome and Letterer-Siwe disease. So consistent have these similarities been that it is now generally conceded that all three diseases represent variants or interrelated expressions of the same fundamental malady. The over-all group has been designated as "Histiocytosis X" by Lichtenstein,4 histiocytosis referring to the common cell type or histological hallmark and X indicating the need