Background : The additional of intrathecal morphine to local anesthetics is highly effective for the management of postoperative pain, however pruritus is a common side effect. The incidence is especially high in patients undergoing cesarean delivery. Objective : We investigated the efficacy of ondansetron in preventing intrathecal morphine-induced pruritus in patients undergoing cesarean delivery. Methods : In a randomized, double-blinded, placebo-controlled study, eighty-six parturients undergoing elective cesarean delivery with spinal anesthesia were studied. All parturients received 0.5% hyperbaric bupivacaine and 0.2 mg of morphine in trathecally. After a child birth, the patients were randomized to receive 8 mg of ondansetron or normal saline (placebo) intravenously, Both groups were evaluated postoperatively for the incidence and the severity of pruritus up to 24 hours. Results : The overall incidence of pruritus was significantly lower in the ondansetron group (53%) (P = 0.01). The postoperative pain score and time to flatus passage were not significantly different. There were no significant complications associated with ondansetron use. Conclusion : Ondansetron prophylaxis significantly reduced the incidence of intrathecal morphin-induced pruritus in patients undergoing cesarean delivery under spinal anesthesia.