We describe two women; one (patient 1) with the diagnosis of acute myeloblastic leukemia (AML), the second (patient 2) with myelodysplastic syndrome (MDS). Both patients underwent allogeneic bone marrow transplantation (BMT), from their HLA-matched brothers. Cytogenetic analysis after the BMT revealed a chromosomal mosaicism in both patients, with the karyotype 46,XX/45,X with no sign of the Y chromosome. The origin of the clone with monosomy X was determined using cytogenetic analysis including heteromorphism and segregation of DNA polymorphic markers. The results led us to the conclusion that in both patients the origin of the 45,X clone was that of the donors. Patient 1 had MDS-like syndrome after the BMT and was stabilized in the chimeric state; to date she is doing well. Patient II also had MDS. However, in her case, it was her primary disease. The graft in patient II was rejected and she died 6 months after BMT.