A patient with acute myeloblastic leukemia received high-dose busulfan (1 mg/kg by mouth every 6 h for 4 days) as myelo-ablative therapy for autologous bone marrow transplantation. Rapid entry of busulfan into the cerebrospinal fluid (CSF) was observed. Plasma and CSF concentrations of busulfan were comparable during the 4 days of treatment. The elimination half-lives of busulfan in plasma and CSF were 2.6 h and 2.8 h respectively during the first 12 h after the last dose.