In order to make an objective evaluation of anti-emetic effect, safety and usefulness of ondansetron injection in nausea and vomiting associated with cancer chemotherapy, we carried out a double-blind placebo controlled comparative study in patients receiving high-single dose (50 mg/m2 or more) of cisplatin. Either 4 mg of ondansetron or saline injection was given intravenously at 15 min. before administration of cisplatin. If anti-emetic effect of the test drug was insufficient, an additional dose of 4mg of ondansetron was given intravenously, as the rescue medication. Ondansetron was significantly superior to placebo in anti-emetic effect (p < 0.001). Efficacy rates were 66.7% (22/33 cases) in ondansetron and 20.0% (6/30 cases) in placebo groups. Seven and 21 cases required rescue medication with 4 mg single intravenous dose of ondansetron due to insufficiency of anti-emetic effect, in ondansetron group and placebo group, respectively. Thus the number of patients who required rescue medication was obviously greater in placebo group than that in ondansetron group. The rates of inhibitory effect of rescue medication on nausea and emesis were 14.3% (1/7 cases) in ondansetron group and 61.9% (13/21 cases) in placebo group. Side effects were observed in 1 case (eruption) in ondansetron group and in 2 cases (headache, diarrhoea; 1 case each) in placebo group. Furthermore, fever developed in 1 case in placebo group after the rescue medication. Elevation of total bilirubin value was observed in 2 cases in ondansetron group and 1 case in placebo group, however, these changes were mild and did not pose noteworthy clinical problem. From these results, ondansetron was shown to possess an excellent anti-emetic effect on nausea and emesis induced by highly emetogenic anti-cancer drugs, such as cisplatin, and to have no problem in safety, and thus it was considered to be a useful anti-emetic agent.