Erdheim-Chester disease (ECD) is a type of histiocytosis that mainly occurs in adults. Adolescents and children are rarely affected. Like all histiocytic diseases, ECD can be challenging to diagnose because it can affect any part of the body and causes a wide variety of symptoms. A biopsy (a sample of the tissue) can be difficult to interpret. Nearly all people with ECD have tumors in their leg bones. About half of them also have pain in the legs or other bones. General symptoms, such as fatigue and weakness, are extremely common in ECD. Problems with balance, thinking, memory, and mood are frequent too. ECD is more likely than the other histiocytic diseases to affect the kidneys, heart, skin around the eyelids, and area behind the eyes. Some people with ECD have mild illness. Others have severe symptoms and difficulty performing normal daily activities. ECD is most serious when it affects the brain. This can cause problems with walking, speech, coordination, and other issues. Treatment is almost always needed for ECD.