We report on a patient with Klinefelter's syndrome who underwent successful syngeneic peripheral blood stem cell transplantation (PBSCT) for chronic myelogenous leukemia (CML). A-46-year-old man was given a diagnosis of chronic phase CML in May 1994 on the basis of findings of leukocytosis (54,000/microliter) and bone marrow chromosomal abnormalities [47, XXY, t(9; 22; 14) (q34; q11; q24)]. Hydroxyurea and interferon alpha were administered. In August 1996, a syngeneic transplant was performed following myeloablative therapy, using peripheral blood stem cells collected from the patient's identical twin brother, who had been pretreated with rhG-CSF. Following transplantation (4.0 x 10(6) CD34+ cells/kg) and the subsequent administration of rhG-CSF, the patient rapidly achieved normal tri-lineage hematopoiesis. A post-transplant chromosomal analysis of the patient's bone marrow cells detected the 47, XXY karyotype. Although the major BCR-ABL gene had been detected in bone marrow by RT-PCR methods prior to the syngeneic PBSCT (August 1996), it was not detected after PBSCT (January 1997). In March 1998, interphase fluorescence in situ hibridization (FISH) procedures disclosed XXY signal patterns in peripheral blood lymphocyte samples from the patient and donor, at frequencies of 96% and 97%, respectively. Both the patient and donor had high levels of serum FSH and LH and low levels of serum testosterone.