In a multicentre, randomized, double-blind, placebo-controlled dose-ranging study, 1030 patients undergoing outpatient surgery with general anaesthesia received i.v. dolasetron mesylate (12.5, 25, 50, or 100 mg) or placebo. The principal outcome measure was the proportion of patients who were free of emesis or rescue medication for the 24-h period after the study drug was given; the subsidiary outcome measure was survival time without rescue medication. Effects on nausea were quantified using a visual analogue scale. Compared with placebo, a complete response was significantly higher when all four dolasetron doses were combined (49% vs. 58%, P = 0.025). In females, dolasetron, 12.5-mg, dolasetron provided maximum clinical benefit (effectiveness compared with adverse events), with no additional benefit in complete response rates or nausea visual analogue scale scores at higher doses. No significant differences were observed in complete response for any dolasetron dose in males compared with placebo. The majority of adverse events reported were mild or moderate. Dolasetron provided well-tolerated, safe, and effective prophylaxis for post-operative nausea and vomiting with maximum effectiveness observed at a dose of 12.5 mg.