BACKGROUND: Loss of sex chromosomes in bone marrow is observed both in elderly persons as an aging phenomenon and in patients with hematologic malignancies. The purpose of this study was to evaluate the incidence and clinical significance of sex chromosome losses in patients with hematologic diseases, comparing the characteristics between patients with sole and secondary sex chromosome losses in conjunction with other chromosomal abnormalities. METHODS: Study group included 868 patients with hematologic diseases between June 1998 and May 2006. The cells of bone marrow aspirates were processed using unstimulated culture methods such as direct, 24-hr and/or 48-hr culture. Sex chromosome losses were included in the karyotype, when X or Y chromosome loss is observed in more than 2 metaphase cells. RESULTS: The sex chromosome losses in bone marrow were found in 5.1% of the patients and 1.8% showed sex chromosome losses as a sole chromosomal abnormality. According to the disease categories, the incidences of sex chromosome losses were as follows: acute myelogenous leukemia (AML), 9.5%; acute lymphoblastic leukemia, 0%; myelodysplastic syndrome, 6.0%; chronic myelogenous leukemia 3.6%; myeloproliferative disorders, 1.3%; multiple myeloma (MM), 13.0%; chronic lymphocytic leukemia, 0%; malignant lymphoma, 3.8%; and benign hematologic diseases 2.2%. The patients with sex chromosome losses as a sole chromosomal abnormality were all male and median age was higher than that of patients with sex chromosome losses as a secondary abnormality (64 vs. 58 yr, P=0.02). The proportion of metaphase cells with sex chromosome losses was significantly lower in patients with sex chromosome losses as a sole chromosomal abnormality (40% vs. 100%, P<0.0001). The changes of sex chromosome loss were correlated with the disease status of AML and MM. CONCLUSIONS: These results suggest that secondary sex chromosome losses in conjunction with other chromosomal abnormalities seem to be one of the clonal abnormalities, whereas sex chromosome losses as a sole change seem to be an aging phenomenon, but further studies are needed.