Etiology and Classification THE ETIOLOGY of agnogenic myeloid metaplasia is unclear as is its relationship to myelofibrosis. This results in confusion regarding the progression of the disease, its medical therapy, and the indications for splenectomy. Agnogenic myeloid metaplasia is a term originated by Jackson et al to describe a condition in which the spleen is active in extramedullary hematopoiesis.12 The peripheral blood resembles somewhat that of chronic myelogenous leukemia but the bone marrow does not have the typical invasive and neoplastic character of leukemia. In fact, the marrow picture may vary from hyperplastic to hypoplastic and varying degrees of fibrosis may be present. It appears that the bone marrow morphology changes in this sequence, and myelofibrosis is the term used to describe the end stages with complete replacement of the bone marrow space with fibrous tissue. Splenic red blood cell production is an integral part of agnogenic myeloid metaplasia.