The incidence of postoperative nausea and vomiting (PONV) is high in women during menstruation. We have compared the efficacy of droperidol, metoclopramide and granisetron in the prevention of PONV in female patients during menstruation undergoing major gynaecological surgery. In a randomized, double-blind study, 120 patients received droperidol 25 micrograms kg-1, metoclopramide 0.2 mg kg-1 or granisetron 40 micrograms kg-1 (n = 40 in each group) i.v. immediately before induction of anaesthesia. A standard general anaesthetic technique and postoperative analgesia were used throughout. There was a complete response, defined as no PONV and no administration of rescue medication, during the 24-h observation period in 45% of patients in the droperidol group, 38% in the metoclopramide group and 70% in the granisetron group (P = 0.021 vs droperidol, P = 0.003 vs metoclopramide). There was no difference in the incidence of adverse events between groups. We conclude that the prophylactic antiemetic efficacy of granisetron was superior to that of droperidol or metoclopramide for prevention of PONV in women during menstruation.