In a prospective, randomized, double-blind, placebo-controlled, multicentre study, the efficacy of prophylactic tropisetron (2 mg) or ondansetron (4 mg) for the prevention of post-operative nausea and vomiting after abdominal or non-abdominal surgery with general balanced anaesthesia was studied in 842 ASA I-III patients. In patients undergoing abdominal surgery, ondansetron and tropisetron reduced the frequency of emetic episodes compared with the placebo (29%, 30% vs. 42% respectively). In men, neither tropisetron nor ondansetron had an effect different from the placebo, whereas in women both drugs led to lower rates of emetic episodes and nausea. In comparison with abdominal surgery, fewer patients in the non-abdominal surgery subgroup had emetic episodes (42% vs. 23% in the placebo group). However, neither tropisetron nor ondansetron was significantly different from the placebo in this patient subgroup. In conclusion, for patients at increased risk of post-operative nausea and vomiting, a prophylactic therapy at the lowest effective dose with tropisetron or ondansetron may be useful.