The pharmacokinetics of high-dose busulphan was studied in 17 patients during conditioning prior to bone marrow transplantation using deuterium-labeled busulphan (d8-BU). About 50% of busulphan doses 1 and 16 was replaced with d8-BU. Patients were treated with phenytoin or diazepam as prophylactic anticonvulsant therapy. Patients who received phenytoin demonstrated significantly higher clearance (mean +/- SD, 3.32 +/- 0.99 ml min-1 kg-1), a lower area under the concentration-time curve (AUC, 5,412 +/- 1,534 ng h ml-1; corrected for dose/kilogram) and a shorter elimination half-life (3.03 +/- 0.57 h) for the last dose of d8-BU (dose 16) as compared with the first dose (2.80 +/- 0.78 ml min-1 kg-1, 6,475 +/- 2,223 ng h ml-1 and 3.94 +/- 1.10 h, respectively). No difference in the above mentioned pharmacokinetic parameters was seen in patients treated with diazepam. Moreover, a continuous decrease in the steady-state level of busulphan was observed in four of seven patients in the phenytoin-treated group, whereas in the diazepam group, such a decrease was seen in only one of eight patients. We conclude that phenytoin used as prophylactic anticonvulsant therapy alters busulphan pharmacokinetics and, most probably, its pharmacodynamics. For adequate prophylactic therapy, anticonvulsants with fewer enzyme-inductive properties than phenytoin should be used.