Retroperitoneal infiltration as the first sign of Erdheim‐Chester disease

: We report a case of an elderly man with bladder cancer, in whom the first manifestation of Erdheim‐Chester disease was retroperitoneal infiltration detected during routine follow‐up. The disease was diagnosed on the basis of histology and immunochemistry findings (presence of histiocytes) and of imaging findings (plain radiography, computed tomography, magnetic resonance imaging, and bone scintigraphy). The differential diagnosis with respect to other causes of retroperitoneal infiltration is discussed.