Casopitant mesilate is a novel, orally active tachykinin NK(1) receptor antagonist. In preclinical pharmacological testing it revealed potent and long-lasting antiemetic effects both alone and in combination with ondansetron in a ferret model using cisplatin as the emetic agent. Both phase II and phase III studies have shown robust antiemetic activity when casopitant was coadministered with ondansetron and dexamethasone, either as a single oral dose or as a,3-day i.v./oral dose regimen, in patients receiving moderately or highly emetogenic chemotherapy. A quality-of-life study showed significant differences in favor of casopitant and standard antiemetic therapy over standard therapy alone in patients receiving highly emetogenic chemotherapy. The addition of casopitant to standard antiemetic therapy was generally well tolerated; the most common adverse events were fatigue, neutropenia, leukopenia and anemia, similar to those on standard antiemetic therapy alone.