Busulphan (1, 4-bis [methanesulfonyl-y] butane) is a bi-functional alkylating agent that, in combination with cyclophosphamide, has been commonly used in conditioning regimens before hematological stem cell transplantation (HSCT) for nearly 20 years. Busulfan has a very narrow therapeutic index, and acute toxicity may be related to absorption and disposition of the drug and metabolites. Precise delivery of the oral formulation is compromised by erratic gastrointestinal absorption, particularly in infants and small children. An intravenous busulfan formula was approved nearly 40 years after the approval of the oral formulation. Busulfan levels expressed as the area under the concentration-time curve (AUC) higher than 1500 μM * minute were reported to increase the risk of developing veno-occlusive disease (VOD), while low levels may result in engraftment failure or disease relapse. VOD occurs in 11-40% of patients undergoing HSCT and is associated with death in 3.3% of patients. Measurement of individual plasma busulfan levels during oral or intravenous dosing to obtain an AUC is likely to provide the necessary elements to monitor the drug disposition, ensuring efficacy and preventing toxicity of patients undergoing HSCT. It is also important to consider the busulfan drug-drug interactions and adverse drug reactions that can develop during the therapeutic process. Busulfan therapeutic drug monitoring and dose-adjustment should be performed in specialized laboratories staffed by well-trained personnel.