The May 2003 COM. A 57-year-old woman presented with slurring of her speech and right arm weakness. Her past medical history included idiopathic hypertrophic, subendocardial stenosis (IHSS), arthritis, asthma, congestive heart failure, hypertension and NIDDM. Neurological examination showed persistent word finding difficulty but her motor and sensory function had essentially returned to normal. Extensive laboratory studies were unrevealing. Imaging studies showed a meningeal lesion over the left posterior parietal lobe and the findings suggested an infectious or inflammatory process. A biopsy of the involved dura and meninges was performed and revealed leptomeningeal Rosai-Dorfman disease. Emperipolesis was noted. The finding of emperipolesis is characteristic of Rosai-Dorfman disease of the leptomeninges, but in 30% of cases, this feature will not be identified. Large pale histiocytes of Rosai-Dorfman disease are immunoreactive for S-100 protein and KP1, but negative for CD1a. The differential diagnosis of a chronic inflammatory infiltrate containing numerous, large histiocytes includes granulomatous diseases such as Wegener graulomatosis and sarcoid, Hodgkin disease, and Langerhans histiocytosis. CNS Rosai-Dorfman most commonly involves patients between 20- and 40-years-old, with a slight male predominance. Approximately 75% of cases are intracranial, whereas 20% involve the spine. Over 90% of CNS Rosai-Dorfman cases involve the leptomeninges and are seen by neuroimaging as a dural-based, contrast-enhancing masses that often elicit vasogenic edema in the underlying brain. Thus, clinically and radiologically, the disease is thought to represent meningioma. Leptomeningeal Rosai-Dorfman disease is considered a benign condition and in most cases surgical resection is the treatment of choice. Although the number of cases in the literature is small, disease progression following surgical resection is uncommon. Little is known regarding the pathogenesis of Rosal-Dorfman disease. Most have suggested that it represents either an autoimmune disease or a reaction to an infectious agent that has yet to be discovered. Currently it is best considered a benign, idiopathic histiocytosis.