Erdheim–Chester disease is a rare xanthomatosis that may present with characteristic radiologic and histologic features. There have been conflicting reports regarding the nature of this process, including whether it represents a reactive or neoplastic lesion. We present the clinical histories, pathologic findings, and an analysis of clonality using the HUMARA assay in two patients diagnosed with Erdheim–Chester disease. One case has previously been documented in the literature. Histologically, both cases demonstrated sheets of foamy xanthomatous histiocytes with widespread infiltration of the viscera. These regions were punctuated by variable amounts of inflammation, including lymphocytes, plasma cells, and occasional Touton-type giant cells. The histiocytes were immunoreactive for CD68 and CD163; they did not stain with S100 or CD1a. One case was found to be monoclonal; however, the second case had extensive DNA degradation; thus, clonality could not be assessed. In addition to contributing an additional report of this rare disease to the literature, we demonstrate the histiocytes to express CD163, thereby further supporting a monocyte/macrophage basis. Moreover, in confirming clonality, our observations lend additional evidence to the view that Erdheim–Chester disease represents a neoplastic process.