Erdheim-Chester disease (ECD) is a rare condition that can affect many parts of the body. It has been diagnosed in children, but it most commonly affects adults. ECD causes the over-production of immune cells called histiocytes, which then accumulate in tissues and organs in the body. Parts of the body that may be involved include the long bones, retroperitoneum, skin, eyes and eyelids, lungs, brain, heart, kidneys, and pituitary gland; however various other tissues or organs can be affected. The signs and symptoms of ECD vary from person to person depending upon the specific locations and extent of involvement. For example, the long bones of the legs are commonly affected, leading to bone pain. When tissues around the eyes are affected, a person may have soft, fatty bumps on the eyelids or protrusion of the eyeballs. Lung involvement may cause pulmonary fibrosis, which can lead to severe complications. People with pituitary gland involvement may develop diabetes insipidus, and brain involvement may lead to various neurologic symptoms. Non-specific symptoms that may develop in people with ECD include weight loss, fever, muscle and joint aches, and malaise. The underlying cause of ECD is not always known. Some people with ECD test positive for an acquired (non-inherited) genetic change in the BRAF gene (most commonly) or one of several other genes, which may allow histiocytes to reproduce uncontrollably. The disease is diagnosed based on the symptoms, biopsies of involved tissue examined under a microscope (pathology), and specialized tests that may include blood tests and imaging studies such as X-rays, CT scans, MRIs, and bone scans.