Many nonrandom chromosome abnormalities, usually autosomal in nature, have been found to be associated with specific types of acute nonlymphocytic leukemia (ANLL) and myelodysplastic syndromes (MDS). Specific abnormalities involving the sex chromosomes are rare. We have recently identified a structural abnormality of the X chromosome, for example, idic(X)(q13) in three patients: two with MDS progressing to ANLL and one with ANLL de novo. All three patients were elderly females with a very aggressive form of ANLL. Six other patients with a similar abnormality have been discovered in the literature; all having either MDS or ANLL and a short survival. It is suggested that the abnormality identifies a subset of MDS and ANLL occurring in elderly females.