Juvenile dermatomyositis (JDM) is a disease in children that causes skin rash (dermato) and muscle inflammation (myositis). It results in weak muscles.

JDM is a type of autoimmune disease. The immune system is a group of cells that protect the body from infections. In autoimmune diseases such as JDM, these cells fight the body's own tissues and cells. This causes inflammation. Sometimes it causes tissue damage.