This study was conducted to determine the efficacy and safety of four intravenous (IV) doses of dolasetron, an investigational 5-HT3 receptor antagonist, for the treatment of postoperative nausea and/or vomiting (PONV) after outpatient surgery under general anesthesia. This multicenter, randomized, double-blind trial compared the antiemetic efficacy of 12.5, 25, 50, or 100 mg IV dolasetron with placebo over 24 h using complete response (no emetic episodes and no rescue medication), time to first emetic episode or rescue medication, and patient nausea and satisfaction with antiemetic therapy as rated by visual analog scale (VAS). Of 1557 patients enrolled, 620 patients were eligible for treatment. Complete response rates for all dolasetron doses-12.5 mg (35%), 25 mg (28%), 50 mg (29%), and 100 mg (29%)- were significantly more effective than placebo (11%, P < 0.05). There was a significant gender interaction for complete response (P < 0.01). Of the patients in the 25-mg and 100-mg dose groups, 12% and 13%, respectively, experienced no nausea (VAS score < 5 mm) versus 5% in the placebo group (P < 0.05). There were no clinically relevant changes in vital signs or laboratory values and no trends with dose for adverse events. Dolasetron is effective for treating PONV and has an adverse event profile similar to that of placebo. The 12.5-mg dose was as effective as larger doses for complete response. Implications: Nausea and vomiting are common problems for postsurgical patients. In this study of 620 patients undergoing surgery, a 12.5-mg dose of intravenous dolasetron, a new serotonin-receptor blocker, was significantly more effective than placebo in treating established postoperative nausea and vomiting. Dolasetron 12.5 mg was as safe as placebo.