During the past decade the demand for outpatient surgery has grown rapidly. Postoperative nausea and vomiting is one of the more common undesirable consequences of surgery, which may significantly delay the patient’s discharge from the ambulatory surgery center. None of the currently used antiemetic drugs is considered totally effective in abolishing nausea or vomiting. The purpose of this study was to compare the efficacy of ondansetron, a highly selective 5-hydroxy-tryptamine subtype-3 receptor antagonist, with that of metoclopramide for the prevention of postoperative emesis in patients undergoing cataract surgery. The incidence of postoperative nausea was significantly less in the ondansetron group than that in the metoclopramide group (p = 0.046). Although the incidence of vomiting was clinically less frequent in the ondansetron group, there were no significant differences between both treatment groups. To our knowledge, this is the first study to demonstrate that ondansetron is effective to prevent postoperative emesis after extracapsular cataract extractio