Background: Ondansetron is both a central and peripheral serotonin (5HT) receptor antagonist and droperidol is a dopaminergic blocking drug which acts centrally at the chemoreceptor trigger zone. We assessed the efficacy and adverse effects of ondansetron, droperidol or both, in the prevention of postoperative emesis during postoperative intravenous patient-controlled analgesia (PCA) using butorphanol and ketorolac medication. Methods: We studied 60 women, aged 25-60 yrs, who underwent total abdominal hysterectomy (TAH), under general anesthesia using $N_2O-O_2$-enflurane. A bolus dose of 1 mg of butorphanol and 4 mg of ondansetron were given to patients and thereafter, PCA was started using 10 mg of butorphanol and 240 mg of ketorolac mixed into the 5% D/W solution (total volume; 100 ml, 1 ml of bolus dose, and 10 min of lockout interval). We also added ondansetron 4 mg (Group O, n = 20), ondansetron 4 mg and droperidol 2.5 mg (Group OD, n = 20), or droperidol 2.5 mg (Group D, n = 20) to the PCA drug. The severity of pain, nausea, vomiting, sedation and other side effects were assessed at 0, 1, 2, 6, 12, 24, 36 and 48 hr after awakening. Results: There was no difference in the incidence of nausea and vomiting between the three group [Group O: 4 (20%) and 3 (15%), respectively; Group OD: 1 (5%) and 1 (5%), respectively; Group D: 3 (15%) and 3 (15%), respectively]. Group O showed a lower sedation score than the other groups (P < 0.05). The pain score and other side effects did not show any difference between the groups. Conclusions: The combination of ondansetron and droperidol showed no clinical benefit compared with ondansetron or droperidol alone for prevention of postoperative nausea and vomiting during postoperative PCA using butorphanol and ketorolac.