A Gnogenic myeloid metaplasia may best be defined as a disorder involving hematopoiesis outside the usual marrow sites, most frequently accompanied by fibrosis of the marrow. This condition represents a distinct clinical-hematologic-roentgenologic entity, with characteristic features. The literature dealing with this disease is voluminous and confused. References are made to primary and secondary forms and some investigators have even considered it to be a variant of leukemia, including it among the “myeloproliferative disorders.” Synonyms include “pseudoleukemia with splenic-cachexia,” “osteosclerosis with myelofibrosis,” “splenic pseudoleukemia,” “aleukemic myelosis,” “myeloproliferative syndrome,” “myeloid metaplasia,” “panmyelosis,” “megakaryocytic myelosis,” etc. The term agnogenic myeloid metaplasia is becoming increasingly acceptable. It is not within the purview of this presentation to review and evaluate the entire literature. Some of the classic articles which may best be read in the original are those by Jackson, Parker, and Lemon (3); Korst et al. (4); Sussman (8); Windholz and Foster (12); Mulcahy (5); Block and Jacobson (1); Green et al. (2); Wasserman (11); Rosenthal (6); and Rosenthal and Erf (7). Clinical and Hematologic Findings When the patient first presents himself for medical evaluation, he is usually seen with the following clinical and hematologic picture: The history is often vague and nonspecific. Complaints may include fatigue, fullness in the abdomen, and sometimes hemorrhagic symptoms. On physical examination, pertinent findings are usually pallor and hepatosplenomegaly. In the blood work-up, two striking abnormalities are generally found in the peripheral smear which must always suggest the possibility of this disorder. These are: (a) nucleated red cells and (b) young members of the neutrophilic series. Anemia of the normocytic and normochromic type is frequently, but not necessarily, present. The platelet count is variable; it may be normal or reduced, but often there is a striking thrombocytosis. The white count ranges from an extreme leukopenia to leukocytosis, so that a true leukemia may be suspected. Because of the occasional leukocytosis and because of the polycythemic phase that may precede this disorder, there have been attempts to classify the disease with the leukemias. Bone marrow usually cannot be aspirated. Only after surgical removal of a section of bone marrow, or upon biopsy of a section of bone from such areas as the iliac crest, is it possible to establish histologically the presence of a typical myelofibrosis picture, which is the signature of this disorder.