BACKGROUND The administration of low-dose bupivacaine can limit the distribution of spinal block to reduce adverse haemodynamic effects. Intrathecal opioids can enhance analgesia in combination with subtherapeutic doses of local anaesthetics. We aimed at comparing the efficacy of intrathecal fentanyl and sufentanil with low-dose diluted bupivacaine for transurethral prostatectomy (TURP) in elderly patients. METHODS Seventy patients undergoing TURP were randomly allocated into two groups. Group F (n=35) received fentanyl 25 microg+bupivacaine 0.5% (0.8 ml)+normal saline 0.3 ml and Group S (n=35) received sufentanil 5 microg+bupivacaine 0.5% (0.8 ml)+normal saline 0.7 ml--in total, bupivacaine 0.25% (1.6 ml) intrathecally. Onset and duration of the sensory block, the degree of the motor block, side-effects, and the perioperative analgesic requirements were assessed. RESULTS The median peak level of the sensory block was significantly higher in Group S than in Group F (P=0.049). Group S required fewer perioperative analgesics than Group F (P=0.008). The time to the first analgesic request was longer in Group S (P=0.025). There were no differences between the groups for the onset and recovery time of the sensory block, degree of the motor block, quality of anaesthesia, or adverse effects. CONCLUSIONS Low-dose diluted bupivacaine with fentanyl 25 microg or sufentanil 5 microg can provide adequate anaesthesia without haemodynamic instability for TURP in elderly patients. However, sufentanil was superior to fentanyl in the quality of the spinal block produced.