A progressive, chronic disease in which the bone marrow is replaced by
fibrous tissue and blood is made in organs such as the liver and the spleen,
instead of in the bone marrow. This disease is marked by an enlarged spleen
and progressive anemia. Also called chronic idiopathic myelofibrosis, primary
myelofibrosis, myelosclerosis with myeloid metaplasia, and idiopathic
myelofibrosis.