In a prospective, randomized, multicentre, double-blind, placebo-controlled study, we have compared the efficacy of a single i.v. dose of tropisetron 0.5 mg, 2 mg and 5 mg in the prevention of postoperative nausea and vomiting (PONV). We studied 385 ASA class I and II female patients undergoing abdominal or vaginal gynaecological surgery, including laparoscopy. Tropisetron or placebo were administered before a standardized general anaesthetic. The frequency of vomiting in the 24-h period after entry into the recovery room was reduced from 44% after placebo to 31%, 26% and 30% after tropisetron 0.5 mg, 2 mg and 5 mg, respectively (P = 0.06, P = 0.009 and P = 0.043; unadjusted). Compared with placebo, nausea was reduced from 55% to 46%, 34% and 46% (P = 0.25, P = 0.003, P = 0.22), and need for rescue treatment from 39% to 29%, 23% and 35% (P = 0.13, P = 0.017 and P = 0.59) for the same groups. Tropisetron 2 mg appeared to be the optimal dose for prophylaxis against PONV with a side-effect profile similar to that of placebo.