A 54-year-old woman with chronic myelogeneous leukemia was admitted to our hospital on February 15th, 2001 to undergo allogeneic bone marrow transplantation (BMT). We started the transplantation preconditioning with busulfan and high-dose cyclophosphamide on February 22nd, 2001. However, symptoms of a psychiatric nature, such as hallucination, persecution complex, auditory hallucination and sleeplessness, occurred by the third day of treatment with busulfan. Thus, we decided to discontinue conditioning and stopped the administration of BMT at that point. However, pancytopenia persisted for more than 20 days. She finally underwent BMT followed by reduced-intensity conditioning with fludarabine and ATG from a sex-mismatched, HLA-identical sibling donor on April 19th, 2001. To prevent any exacerbation of the psychotic symptoms, the patient was hospitalized in a laminar flow instead of a bio-free room. Graft-versus-host disease occurred on the 32nd hospital day, and was brought under control by steroid treatment. Achievement of complete chimeras was confirmed on the 54th hospital day. Her mental condition was kept stable with antidepressant drugs and tranquilizers, although minor changes in the combination of drugs were required to treat transient exacerbation of psychosis after a short period at home. She was discharged on September 1st, 2001. We think that non-myeloablative stem cell transplantation is a useful treatment for patients with hematological malignancy complicated with psychiatric disorders.