Summary. We describe 10 cases of primary myelodysplastic syndrome in which marrow fibrosis was striking at presentation. All the cases showed trilineage dysplasia with increased megakaryopoiesis and marked reticulin fibrosis. Significant organomegaly was notably absent. This association has hitherto not been highlighted and it is important to distinguish these cases from those of idiopathic myelofibrosis with which they may be confused. Furthermore, their comparatively long survival distinguishes these cases from those previously described as acute myelodysplasia with myelofibrosis and malignant myelosclerosis. The pathogenesis of fibrosis in these cases may be related to disordered megakaryopoiesis and the platelet-derived cytokines that may be released. The treatment of these fibrotic cases remains problematical and further investigation is required.