A very rare disorder in which too many histiocytes (a type of white blood cell) build up in certain tissues and organs and damage them. Erdheim-Chester disease most often affects the long bones of the arms and legs, but it can also affect the skin, lungs, heart, kidneys, brain, tissue behind the eyes, and the retroperitoneum (the back wall of the abdomen). The symptoms can range from mild to severe and life-threatening. Erdheim-Chester disease is most common in adults. It is a type of histiocytosis. Also called ECD.