Busulfan is an alkylating agent that is widely used in preparative regimens for bone marrow transplantation (BMT). We developed a high-performance liquid chromatographic (HPLC) assay for the determination of plasma busulfan concentrations in 30 patients who received oral doses of 1 mg/kg. Concentrations were fit by a one-compartment pharmacokinetic model with first-order absorption. The pattern of absorption and elimination varied widely between patients, with peak concentrations ranging from 1.2 to 10.4 mumol/l (mean, 4.25 +/- 2.49). The elimination half-life ranged from 58 to 433 min (harmonic mean, 140 min). The AUC contributed by a single oral dose ranged from 606 to 5,144 mumol-min/l (mean, 2,012 +/- 1,223). Patients were evaluated for the development of veno-occlusive disease (VOD), a treatment complication that occurs in 20% of patients undergoing BMT and causes 10% of transplantation-related deaths. All six patients who developed VOD had an AUC greater than the mean, and five of them had an AUC that was greater than 1 SD above the mean. The occurrence of VOD was highly correlated with an increased AUC (greater than 1 SD above the mean) (X2 = 18; P less than 0.0001). Using multivariate logistic regression, no other statistically significant pharmacokinetic predictor of VOD was found. The tenfold variability in the busulfan AUC and the statistical association of increased AUC with the development of VOD suggest a possible role for therapeutic monitoring in this setting.