Erdheim-Chester disease (ECD) is a rare, distinct clinicopathologic entity with nearly pathognomonic radiographic features. The lesions consist of lipid-storing CD68 (+), CD1a (-) non-Langerhans' cell histiocytes, either localized to the bone or involving multiple organ systems in the body. Whether these histiocytic proliferations represent monoclonal neoplastic populations or are part of a polyclonal reactive process is unclear. We present a case report of ECD in a 35-year-old African-American woman with a progressive course over 6 years. We investigated the clonality of the histiocytes using the HUMARA assay on paraffin-embedded tissue sections but did not find any evidence that these cells represent a monoclonal population. In this report, the characteristics of ECD are reviewed, the genetic basis of the HUMARA assay is discussed, and our results in the context of other clonality investigations reported in the literature to date are summarized.