The authors report a possible interaction between ketobemidone and busulfan during myeloablative treatment of a patient with acute myeloid leukemia. At the time of admission, the patient was receiving ketobemidone 1,000 mg/d as analgesic for a rectal fissure. The patient started conditioning prior to bone marrow transplantation with busulfan (1 mg/kg x 4 for 4 days). High busulfan plasma concentrations were observed after the first dose and the next doses were reduced to 0.7 mg/kg. The kinetics of both drugs revealed that an increase in ketobemidone concentration was followed by an increase in busulfan levels. Substituting ketobemidone with morphine resulted in a decrease in busulfan concentration despite increasing the dose once more to 1 mg/kg.