From 1969--1975 patients with predominantly megakaryocytic myelosis were observed. Diagnosis was made up after histologic examination of iliac crest bone biopsy specimens embedded in methacrylate. From 6 patients rebiopsies and from further eleven ones the removed spleens were available. Autopsy was performed in 12 cases. In our material predominantly megakaryocytic myelosis was especially observed in male patients of advanced ages. The course of the disease was rarely acute, most often chronic and associated with the development of an extreme splenomegaly. Transition into osteomyelofibrosis or osteomyelosclerosis was frequent, the latter condition tended to develop a so-called myelosarcoma. The cause of death often was a generalized hemorrhagic diathesis with gastrointestinal hemorrhage or an fatal infection.