Although the osseous changes now recognized as characteristic of eosinophilic granuloma have been known for many years, this condition was not established as a clinical entity until 1940. In that year Lichtenstein and Jaffe wrote as follows: Eosinophilic granuloma of bone presents itself as a rather well localized, single lesion, starting in the medullary cavity and tending to erode, expand, and perforate the cortex in the bony site affected. When first observed the lesion may already have perforated the cortex and extended into the neighboring soft tissues. Indeed, the roentgenographic and clinical findings may lead one to suspect the presence of a malignant bone tumor. Surgical exploration shows that the affected portion of the bone has been extensively replaced by a more or less soft, yellowish or brownish tissue. In its microscopic appearance the latter is characterized essentially by the presence of compacted, tumor-like aggregates of large phagocytic cells with