Nine cases of myelodysplastic syndrome with a deletion of the long arm of chromosome #11 (11q−) showed ringed sideroblasts, and three of which had an acquired sideroblastic anemia according to the criteria of the FAB classification. In contrast, among four cases of myelodysplastic syndromes with translocation of extra material to the long arm of chromosome #11 (11q+), only one showed bone marrow sideroblasts. These results strongly indicate that an 11q− chromosome is a marker of iron overload in myelodysplastic syndromes. Within the cases of 11q− associated with sideroblastosis, two cytogenetically different anomalies (i.e., terminal or interstitial deletions) were delineated.