A 67-year-old female was admitted to our hospital because of pancytopenia. Forty-six percent of erythroblasts in the bone marrow were ringed sideroblasts. Laboratory findings showed an IgG-kappa monoclonal gammopathy. She was diagnosed as having sideroblastic anemia associated with multiple myeloma in mosaic (45, X/46, XX/47, XXX) Turner's syndrome. There was no response to therapy. The chromosomal pattern of the patient was varied, and was accompanied by the development of refractory anemia with an excess of blasts from refractory anemia with ringed sideroblast 4 months after presentation. Cytogenetic studies suggested that the abnormal clone was restricted to the monosomic cell line.