In a retrospective study, 45 (19.4%) out of 232 patients with MDS revealed myelosclerosis (MS) in bone marrow biopsy (BMB). Histological classification according to FAB criteria showed the following distribution: RA 21 (47%), RARS 1 (2%), RAEB 9 (20%), RAEB-T 3 (7%), and CMMol 11 (24%). Sclerosis occurred in all subtypes of MDS, but with a higher incidence in CMMol. Clinical data showed lower values of hemoglobin and lower platelet counts in MDS.MS. Life expectancy was reduced to 7.8 months, compared with 15.0 months in MDS without MS (p = 0.0026). In RA, the survival times were 9.7 months in MDS.MS, compared to 27.9 months in MDS without MS (p = 0.0035). 21 (47%) of the patients with MDS.MS experienced a transformation into ANLL. Myelosclerosis therefore seems to herald a bad prognosis.