The study of the evolution of 168 cases of idiopathic myeloid splenomegaly allowed to point out the prognostic value of the clinical hematologic, isotopic and radiological parameters. The correlation of these data with the histological type of the osteomedullary lesions at the time of the diagnosis, allows to confirm the long survival of the hyperplastic formes (type I) and the more reserved prognosis in advanced myelofibrosis (type II and III together). Indeed, median survival is of 82 months for type I, and 60 months for type II and type III together. It seems advice to gather the two last types and so, to compare two anatomoclinical entities : one is characterized by hyperplastic marrow with reticulinic fibrosis, usually associated with enlarged spleen and sometimes even with increase red blood cells volume but without radiological bone lesions; the other one is characterized by a marrow of middle importance or marked marrow depletion, fibrosis, with or not osteosclerosis, and shows moderate or marked radiological lesions (respectively 16,9% of the cases) and, often, pancytopenia.