Postoperative nausea and vomiting (PONV) is common; ondansetron is often used as prophylaxis or for breakthrough episodes. Vestipitant is a neurokinin 1 (NK-1) receptor antagonist that is effective for prophylaxis, but its efficacy for treating established PONV is unknown. This study was performed to evaluate the efficacy and safety of vestipitant, compared with ondansetron for the treatment of breakthrough PONV in patients who had already received prophylactic ondansetron before surgery.