Imatinib mesylate (Gleevec, Glivec), formerly STI571; Novartis Pharmaceuticals) is an inhibitor of the Bcr-abl tyrosine kinase that is central to the pathogenesis of chronic myeloid leukaemia (CML). The remarkable results of imatinib mesylate in clinical trials have rapidly and profoundly changed the management of patients with CML. This article will review the development of this molecularly targeted agent. The clinical trials with imatinib mesylate will be summarised along with the pharmacology of this agent. Despite the impressive responses seen in chronic-phase patients, numerous questions remain. For example, how durable will responses to imatinib mesylate be and is it necessary or possible to improve upon these results? Ongoing efforts to address these issues will be discussed.