Erdheim-Chester disease is a rare nonfamilial histiocytic disorder of unknown etiology with characteristic long bone findings. The 3-year survival rate for patients with Erdheim-Chester disease is 50%. Approximately 50% of patients have disease involvement in other tissues, including skin, retro-orbital and periorbital tissues, pituitary-hypothalamic axis, heart, kidney, retroperitoneum, breast, skeletal muscle, and sinonasal mucosa; about 20% of patients have lung involvement. Prognosis generally depends on the extent of the extraosseous disease. For patients with lung involvement, gender distribution is equal, but men typically present at an older age than do women. Approximately 80% of patients present with dyspnea, and most patients have diffuse interstitial infiltrates and pleural and/or interlobar septal thickening on chest radiology. Characteristic lung histopathology includes the accumulation of histiocytes with variable amounts of fibrosis and a variable lymphoplasmacytic infiltrate in a lymphangitic distribution. Immunostains are diagnostically useful, showing immunopositivity for CD68 and factor XIIIa and immunonegativity for CD1a. Birbeck granules are uniformly absent ultrastructurally.