Ondansetron (OND) is a new 5-HT3 receptor antagonist that give complete protection from emesis/nausea in approximately 50% of cisplatin (CDDP)-treated patients. To evaluate if dexamethasone (DEX) added to OND increases antiemetic efficacy, we carried out a double-blind randomized crossover study to compare the antiemetic activity of OND with OND plus DEX. One hundred two chemotherapy-naive patients (44 women and 58 men) scheduled to receive CDDP chemotherapy at doses greater than or equal to 50 mg/m2 entered the study. Eighty-nine patients completed both cycles with the following results: complete protection from emesis/nausea was obtained in 57/59 patients (64.0%/66.3%) with OND and in 81/79 (91.0%/88.8%) with OND plus DEX (P = .0005/P = .0021). At the end of the study, 53% of the patients expressed a treatment preference, and of these, 74% chose OND plus DEX compared with 26% who preferred OND alone, a statistically significant difference (P less than .003). Side effects were very mild and not significantly different between the two treatments. We conclude that OND plus DEX is more efficacious than OND in protecting patients from CDDP-induced emesis and nausea.