Structural abnormalities of the X chromosome are rarely found in neoplastic disorders. We describe three patients with a history of idiopathic acquired sideroblastic anemia (IASA); each one had an abnormal clone of cells in the bone marrow, characterized by a structurally abnormal X chromosome. In two of these patients, the predominant karyotype was 47,X,2idic(X)(q13); in the other patient, it was 46,X,t(X;11)(q13;p15). Inasmuch as all three of these cases involved chromosome band Xq13, as did two previously published cases, we suggest that band Xq13 may be more prone to structural rearrangement than other X chromosome bands in hematologic disorders. The common Xq13 chromosome breakpoint and clinical presentation (IASA) among these three patients and the occurrence of an X-linked type of sideroblastic anemia may suggest that an association exists between X chromosome abnormalities and IASA. Perhaps alteration of a gene or chromosome structure in or near band Xq13 predisposes to development of IASA. The fact that two of these patients had preleukemia and the third had overt acute leukemia may imply that patients with IASA and X chromosome abnormalities have a poor prognosis. Cases of IASA without associated X chromosome abnormalities are known; thus, if an association between IASA and an abnormal X chromosome does exist, most likely it involves only some patients with IASA.