Twenty-four patients with severe post-chemotherapy emesis ( > 15 emetic episodes) refractory to prior combination antiemetic therapy were treated with a selective 5-HT3 receptor antagonist ondansetron (GR38032F). Ondansetron was given as 8 mg three times daily orally for 5 days with the first dose given 1 h prior to chemotherapy. Control of emesis was evaluated over the 5-day period. All chemotherapy was administered on an outpatient basis. Worst day analysis of antiemetic response was 87.5%: complete protection in 9/24 patients (37.5%) and major protection (1–2 emetic episodes) in 12/24 patients (50%). No protection from emesis was observed in 3 patients (12.5%). No side effects and no alterations in liver function tests were observed. Ondansetron is a safe and highly effective antiemetic agen