[Histiocytosis X: a controversial term].

In reference to Histiocytosis X as a controversial term, and analysis is submitted to underline lack of linkage between clinico-histological aspects in the Letterer-Siwe (LS) and Hand Schüller Christian (HSC) alterations. As pointed out, the HSC denomination tends to disappear, while debate becomes more difficult around the LS phenomena. A definition is offered to delimit more precisely the two clinical entities in question.