This review discusses the development and uses of imatinib mesylate, a protein tyrosine kinase inhibitor useful in the treatment of chronic myelogenous leukemia and gastrointestinal stromal tumors that was recently approved by the Food and Drug Administration. Imatinib targets platelet-derived growth factor receptor, inhibits the fusion product of the Philadelphia chromosome, and targets c-kit, a protein tyrosine kinase. The drug may also be effective in the treatment of other tumors that express platelet-derived growth factor receptor or c-kit.