BACKGROUND: The combination of local anesthetics and opioids in the spinal anesthesia has a synergic analgesic effect. Therefore, we compared intraoperative conditions and clinical effects of low dose (5 mg) bupivacaine added with fentanyl versus a conventional dose(10 mg) of bupivacaine in spinal anesthesia for a transurethral resection of the prostate (TURP). METHODS: Forty five ASA class 1 and 2 patients scheduled for a TURP were randomly divided into three groups. Group B (n = 15) received bupivacaine 10 mg, group BF (n = 15) received bupivacaine 5 mg added with fentanyl 20microgram, and group BFE received bupivacaine 5 mg added with fentanyl 20microgram and 0.2 mg epinephrine. Blood pressure and heart rate were recorded every 5 minutes before and after spinal anesthesia. Sensory blockade was measured by a pin-prick test and motor blockade was evaluated by the Bromage motor scale. Side effects including pruritus, nausea, vomiting, respiratory depression, and intraoperative pain were observed. RESULTS: The duration of sensory and motor blockade in group BF was significantly shorter than in group B and group BFE (P<0.05). Without statistical significance, hypotension (6.7%) and bradycardia (13.3%) were observed in group B and pruritus (26.7%) and nausea (13.3%) occurred in all patients who received fentanyl. CONCLUSIONS: The addition of fentanyl 20microgram to low dose bupivacaine 5 mg resulted in short lasting motor and sensory block, compared with conventional dose bupivacaine 10 mg. Therefore, we concluded that the clinical application of this method could provide adequate analgesia with early discharge and no serious side effects for TURP patients.