Xanthomas were divided into hypercholesterolemic and normocholesterolemic forms by Thannhauser and Magendantz.1 This subdivision, founded on serum biochemical determinations, has remained for 21 years the standard classification of xanthomatous lesions. By contrast, chemical analysis of the affected tissue, particularly in the normocholesterolemic group, has been curiously neglected. In xanthoma disseminatum, Hand-Schüller-Christian disease, Letterer-Siwe disease, and eosinophilic granuloma of bone, the serum-lipid levels are normal. The last three are now generally accepted as variants of a single pathological process which Lichtenstein2 referred to as "Histiocytosis X." A variety of other all-embracing names for this entity has been suggested, including eosinophilic granuloma syndrome,3 "Letterer-Christian" disease,4 histiocytic type of reticuloendotheliosis,5 and granulomatous reticuloendotheliosis.6 Other possibly related conditions are juvenile xanthoma and McDonagh's7 naevoxanthoendothelioma which Polano8 and Nomland9 regard as separate entities. On the other hand, Thannhauser,3 Crocker10 and Calnan11 consider them to be similar to