This prospective, randomised, placebo-controlled, double-blind study was performed to evaluate the effects of ondansetron on nausea and vomiting during elective Caesarean section under spinal anaesthesia. Seventy-four full-term parturients were studied. After umbilical-cord clamping, ondansetron 4 mg or 0.9% saline was injected intravenously and the study period continued for 2 h. The severity of nausea was graded from 0 = none to 10 = maximum, while that of vomiting was graded from single = once per min to continuous = multiple per min. The incidence of vomiting was lower following ondansetron (36%) compared with the control group (58%, p < 0.028). Neither the severity of vomiting nor the incidence of nausea was influenced by ondansetron. However, for those who experienced nausea, ondansetron reduced its severity (p = 0.05). We conclude that the intra-operative administration of 4 mg ondansetron intravenously during Caesarean section under spinal anaesthesia significantly reduces the incidence of vomiting and the severity of nausea.