Allogeneic HSCT is a cost-intensive procedure. Oral busulfan (Bu) as part of BuCy2 is a commonly used conditioning regimen but is associated with high plasma variability. IV Bu has more predictable pK parameters allowing better targeting of plasma exposure and reducing hepatic veno-occlusive disease (HVOD) occurrence, related to blood over-exposure. OBJECTIVES: To estimate costs of IV versus Oral Bu-based BuCy2 conditioning in the UK NHS system. METHODS: A simulation based on the strong correlation established between Bu blood over-exposure and the occurrence of HVOD (Kashyap A, BB&MT 2002) included costs of drugs and local HVOD management. RESULTS: The cost of a full course of oral Bu (1mg/kgx16) and IV Bu (0.8mg/kgx16) is £ 116.5 and £ 3220 respectively. Patients receiving oral Bu have a greater risk of developing HVOD (20% vs 5%, p = 0.03, Kashyap 2002). In Southampton the cost of managing HVOD was estimated to be £ 11,050 per case (based on additional in-patient stay, drug treatment and medical management), generating an extra cost per patient of £ 2210 (£ 11,050 x 0.2) and £ 552.50 (£ 11,050 x 0.05) respectively. Therefore the total estimated cost using oral Bu compared to IV Bu in BuCy2 is £ 2326.50 and £ 3772.50 respectively. From an initial cost ratio of 1/27.6 in favor of oral Bu (drug costs only) the ratio dropped to 1/1.6 when the cost of HVOD management was included in the simulation. CONCLUSION: The additional cost of £1,446 with IV Bu is relatively modest in the context of better targeting of the therapeutic window, potential superior transplant outcome, and better time allocation of health care personnel (easier administration, patient monitoring and treatment compliance) seen with Busilvex®.