Imatinib mesylate is a selective and potent small-molecule inhibitor of tyrosine kinases, including Kit, platelet-derived growth factor receptor, and the BCR-Abl fusion protein. Kit plays an important role in gastrointestinal stromal tumours (GISTs) and is one of the most exciting therapeutic targets discovered so far. Clinical trials have consistently shown the dramatic efficacy of imatinib mesylate in patients with GIST. This article will review the development and pharmacology of this small-molecule inhibitor and summarise the clinical trials of imatinib mesylate for the treatment of GIST. Although imatinib mesylate has significantly improved the outcomes of most patients with advanced GIST, unanswered questions remain: what is the role of imatinib mesylate in the pre- and postoperative settings? What is the mechanism of the antitumour activity of imatinib? How do you manage patients whose tumours are refractory to imatinib mesylate?