BACKGROUND: Subarachnoid block is widely used for cesarean section due to the rapid induction, the complete analgesia, the low failure rate and the prevention of aspiration pneumonia. The addition of intrathecal opioids to local anesthetics seems to improve the quality of analgesia & prolong the duration of analgesia. Therefore we compared the effects of fentanyl 20 microg and sufentanil 2.5 microg, which were added to intrathecal hyperbaric bupivacaine. METHODS: Seventy two healthy term parturients were randomly divided into three groups: Group C (control), Group F (fentanyl 20 microg) and Group S (sufentanil 2.5 microg). In every group, 0.5% heavy bupivacaine was added according to the adjusted dose regimen by Harten et al. We observed the maximal level of the sensory block and motor block, the quality of intraoperative analgesia, the duration of effective analgesia and the side effects. RESULTS: There were significant differences between the control and the fentanyl 20 microg and sufentanil 2.5 microg groups for the degree of muscle relaxation, the quality of intraoperative analgesia, the maximal sedation level and the duration of effective analgesia. The frequencies of side effects such as nausea and pruritis in the opioid groups were higher than those in the control group. But there were no differences between fentanyl 20 microg and sufentanil 2.5 microg for the frequencies of nausea and pruritis. CONCLUSIONS: The addition of fentanyl 20 microg or sufentanil 2.5 microg for spinal anesthesia provides adequate intraoperative analgesia without significant adverse effects on the mother and neonate.