Imatinib mesylate (Gleevec/Glivec, Novartis, Basel, Switzerland), formerly called STI571, is a specific and potent inhibitor of the BCR-ABL tyrosine kinase, the molecular hallmark of chronic myeloid leukaemia. Several clinical trials have demonstrated the efficacy of imatinib in different phases of this disease. On the other hand, imatinib is also active against other tyrosine kinases, such as ABL, the stem cell factor receptor (c-kit) and the platelet-derived growth factor receptor, whose inhibition might have potential implications for the treatment of several malignancies. In this regard, imatinib has already shown a remarkable activity in patients with hypereosinophilic syndrome and gastrointestinal stromal tumours. Imatinib is an example of how a better understanding of the pathogenetic mechanisms of a neoplastic disease can lead to the development of a molecular-targeted therapy.