Myelofibrosis is said to occur when there is a formation of fibrous tissue or fibrosis within the bone marrow. Idiopathic myelofibrosis is the term used when the cause of the fibrosis within the bone marrow is unknown. Myelofibrosis may be a reaction to injury that results in the death of bone marrow tissue. In some instances, it is associated with metastasis or the spread of cancer to the bone marrow, metabolic disorders, or chronic myeloid leukemia. Idiopathic myelofibrosis affects both men and women equally with onset usually in adulthood. Myelofibrosis rarely occurs in children; when it does, it is most likely in connection with Albers-Schonberg disease, which affects bone (National Organization for Rare Disorders [NORD], 2000). In two cases, idiopathic myelofibrosis was found to be familial with presentation in two sisters who were 4 years and 7 months at age of onset respectively. There are no indications that idiopathic myelofibrosis occurs in one ethnic group more frequently than in any other groups.