Emesis and nausea are common toxicities seen during high-dose interleukin-2 (IL-2) therapy (720,000 IU/kg i.v. every 8 h). A growing list of randomized studies have documented the efficacy of ondansetron, a potent antagonist of the 5-hydroxytryptamine3 receptor, in preventing acute chemotherapy-induced emesis and nausea. However, no study has evaluated the efficacy of ondansetron in preventing IL-2-induced emesis and nausea. This double-blinded, randomized trial was performed to compare the antiemetic and antinausea efficacy of ondansetron with that of droperidol, a butyrophenone, in patients receiving high-dose IL-2 on protocols at the National Cancer Institute. Ondansetron or droperidol was given intravenously, 30 min prior to the first dose of IL-2 and then every 8 h for the duration of IL-2 treatment. No significant differences were seen between the two agents in complete freedom from emesis (p2 = 0.51), level of nausea (p2 = 0.17), antiemetic treatment failure (p2 = 0.89), and time to first emetic episode (p2 = 0.44). Equivalent doses of IL-2 were administered on each arm of the study, with a similar incidence of liver dysfunction (p2 = 0.15) and diarrhea (p2 = 0.64). Finally, there was no significant difference in the response rates to metastatic disease in either arm of the antiemetic study (p2 = 0.67), and these response rates were similar to those in other patients treated under immunotherapy protocols in the Surgery Branch of the National Cancer Institute with high-dose IL-2. We conclude that droperidol is equally effective in preventing emesis and controlling nausea when compared with ondansetron for patients receiving high-dose IL-2.