Bone marrow from an 81-year-old male with acquired idiopathic sideroblastic anemia was found to be mosaic for 45, X/46, XY cell lines. Analysis of ringed sideroblasts for the presence or absence of a quinicrine fluorescent Y body indicated that all sideroblastic cells had lost the Y chromosome. The demonstration that the ringed sideroblasts were cytogenetically abnormal in this patient provides evidence that the cytogenetic changes often found in patients with sideroblastic anemia may not be due to randomly acquired chromosome aberrations accompanying tissue aging unrelated to the disease process.