Imatinib mesylate (Gleevec), Glivec, formerly STI571; Novartis Pharma AG; Basel, Switzerland) is a rationally-designed, molecularly-specific oral anticancer agent that selectively inhibits several protein tyrosine kinases central to the pathogenesis of human cancer. It has demonstrated remarkable clinical efficacy in patients with chronic myeloid leukemia and malignant gastrointestinal stromal tumors. Treatment with imatinib is generally well tolerated, and the risk for severe adverse effects is low. Adverse effects most commonly include mild-to-moderate edema, nausea and vomiting, diarrhea, muscle cramps, and cutaneous reactions. Hepatic transaminase level elevations and myelosuppression occur less frequently and resolve with interruption of imatinib therapy. In general, the incidence and severity of adverse effects tend to correlate with imatinib dose and, in chronic myeloid leukemia patients, the phase of disease; but, patient age and other factors are also associated with some types of reactions. With prompt and appropriate intervention, adverse effects in imatinib-treated patients have proven to be manageable across the spectrum of severity, and they seldom require permanent cessation of therapy. Dose reduction is not usually necessary, and reduction to subtherapeutic levels is not recommended.