Erdheim-Chester disease (ECD) is a rare, multisystem disorder of macrophages. Patients manifest with histiocytic infiltrates that lead to xanthogranulomatous lesions in multiple organ systems. The cytologic features of this disorder are not well characterized. As a result, the cytologic diagnosis of ECD can be very challenging. The aim of this report is to describe the cytomorphology of ECD in a patient presenting with a retroperitoneal soft tissue lesion. A 54-year-old woman with proptosis and diabetes insipidus was found on imaging studies to have multiple intracranial lesions, sclerosis of both femurs and a retroperitoneal soft tissue mass. Fine needle aspiration (FNA) and a concomitant core biopsy of this abnormal retroperitoneal soft tissue revealed foamy, epithelioid and multinucleated histiocytes associated with fibrosis. The histiocytes were immunoreactive for CD68, CD163, Factor XIIIa and fascin, and negative for S100, confirming the diagnosis of ECD. ECD requires a morphologic diagnosis that fits with the appropriate clinical context. This case describes the cytomorphologic features of ECD and highlights the role of cytology in helping reach a diagnosis of this rare disorder.