High dose busulfan is widely used in preparative regimens for bone marrow transplantation. We describe three cases of accidental busulfan overdosing. Two adults received a single dose of 8 and 18 mg/kg busulfan, respectively. Doses of 9 x 4 mg/kg were ingested by a 14-year-old girl, who experienced seizures. In all cases, no severe liver toxicity including veno-occlusive disease was observed. Plasma samples were obtained from two patients. Busulfan plasma concentrations were far above published values after high-dose busulfan treatment. Busulfan was eliminated by a first-order process. All patients survived these high doses of busulfan and successful transplantation was possible. Two patients died from refractory GvHD on days 91 and 80 after transplantation. One patient is alive in remission after an observation time of 18 months. These cases show that busulfan overdosing may occur and pharmacokinetic evaluation is warranted to estimate risk of early and late toxicity.