Marrow fibrosis (MF) is rarely considered in myelodysplastic syndromes (MDS) although the frequency of this complication ranges from 10 to 50 % in the few reports on this issue, and there are no data on occurrence and significance of this complication in the context of the International Prognostic Scoring System (IPSS) and the World Health Organization (WHO) classification of disease. In a retrospective study, diagnostic bone marrow biopsies from a total of 936 patients with MDS were examined for MF and its relevance to the course of disease. Frequency of MF varied markedly between different types of MDS ranging from 3 % (RARS) to 37 % (MDS, therapy-related; WHO classification, P < 0.000005). Risk of MF furthermore correlated with multilineage dysplasia (P < 0.000005). However, there was no obvious correlation to the IPSS or to karyotype abnormalities. The survival time of patients was significantly reduced by about 50 % from 11 (RAEB-1/-2) - 55 (RARS, RCMD-RS) down to 6 (RAEB-1/-2) - 33 months (RARS, RCMD-RS) in median when MF was detected independently of the IPSS and the classification of disease (FAB, WHO; P = 0.0001). We conclude that MF is an unfavorable complication of MDS significantly shortening the survival time of patients independently of the IPSS and the classification of disease.