Acute myelofibrosis is a rare and still ill-defined disease. Based on morphological observation, immunophenotyping and ultrastructural analysis, we support the assumption that acute myelofibrosis is a malignant disorder mainly of the megakaryocytic lineage and is closely related to acute megakaryocytic/blastic leukaemia. Consequently, the 11 patients reported here were treated with aggressive polychemotherapy with combinations including daunorubicin and cytosine arabinoside and 6-thioguanin or VP16-213. 4 complete remissions, 2 partial remissions and 1 minor response were observed. Duration of aplasia was not significantly prolonged. These findings indicate that the use of aggressive polychemotherapy is feasible in acute myelofibrosis and results in a significant number of remissions.