Gastrointestinal stromal tumor (GIST) is the most common mesenchymal malignancy of the gastrointestinal tract. This tumor is resistant to conventional chemotherapy and radiotherapy. Although surgery has been the only effective treatment for GIST to date, it is not enough to manage metastatic GIST. Imatinib mesylate, a KIT tyrosine kinase inhibitor, is an oral agent that has been found to have a dramatic antitumor effect on metastatic GIST with either wild-type or mutant KIT. Although imatinib mesylate has been used in GIST treatment for several years, its use marks a new era of molecular targeting therapy. While several issues remain, they should be clarified by the current clinical trials and associated laboratory studies.