A double-blind, parallel-group study in 189 ovarian cancer patients compared the efficacy of ondansetron 8 mg i.v. (OND) and metoclopramide 60 mg i.v. (MET) both in combination with dexamethasone 20 mg i.v. in the prevention of carboplatin-induced emesis. On day 1, complete or major control of emesis (0-2 emetic episodes) was observed in 97% patients from the OND group compared with 74% patients from the MET group (p < 0.001). Similarly, a worst-day analysis over days 1-3 showed complete or major control of emesis in 87% patients (OND) compared wth 66% patients (MET) (p < 0.001). Similar findings in favour of the OND group were observed for nausea grade on day 1 and the worst-day grade during days 1-3. OND was better tolerated than MET. Fewer patients from the OND group (13%) reported adverse events compared with the MET group (21%). Extrapyramidal type symptoms were observed in 6 (6%) patients from the MET group (paraesthesia, involuntary movement of the jaw and tongue, and restlessness), compared with none from the OND group. Ondansetron plus dexamethasone is a highly effective and well-tolerated treatment and is significantly superior to metoclopramide plus dexamethasone in the prevention of carboplatin-induced emesis.