Endomyocardial fibrosis (EMF) is a progressive disease of unknown origin (idiopathic) that may seriously affect the heart. Its most obvious feature is a gross change in the makeup of the lining of the heart cavities (the endocardium) of one or both of the lower chambers of the heart (the ventricles) leading to the replacement of normal cells with fibrous tissue (fibrosis). This process is progressive and leads to the narrowing (constriction) of the right or left ventricular cavities. It may involve the valves between the chambers of the heart as well as the tendon-like cords that fix the valves to the ventricles (chordae tendineae). Loeffler's disease is a disease of the heart much like endomyocardial fibrosis. Some clinicians regard it as an early stage of EMF, although this idea remains controversial. Loeffler's disease is a rare disorder of unknown origin, characterized by abnormal increases in the number of particular white blood cells (eosinophilia), and like EMF, gross fibrosis of the endocardium, and inflammation of small blood vessels (arteritis).