A multicentre, randomised, double-blind, cross-over trial was done to compare the efficacy and safety of a serotonin receptor antagonist--ondansetron--and dexamethasone in the prophylaxis of acute and delayed emesis and nausea induced by moderately emetogenic non-platinum-containing chemotherapy regimens. Patients were treated as outpatients and received intravenous ondansetron 4 mg or dexamethasone 8 mg before chemotherapy and oral maintenance (ondansetron 4 mg every 6 h and dexamethasone reducing from 4 mg to 1 mg 6-hourly between days 1 and 5) for 5 days. 112 patients were treated (38 men, 73 women, 1 with no gender recorded; age range 30-73 years) and 100 were evaluable for cross-over analysis. Patients taking ondansetron or dexamethasone reported no significant difference in complete and major control of acute (83% vs 79%, p = 0.46) or delayed (82% vs 88%, p = 0.214) emesis (vomiting plus retches). Significantly more patients on dexamethasone (87%) than on ondansetron (72%) reported control of delayed nausea (days 2-5) (p = 0.003). Both drugs were well tolerated with no significant difference in the number of adverse events, and this is reflected by similar patient preference for ondansetron (40%) and dexamethasone (30%) (p = 0.244). Both drugs offer adequate out-patient control of chemotherapy-induced emesis; however, dexamethasone has an advantage in the control of delayed nausea, and also in terms of cost and resource allocation.