This study investigated whether the addition of 25 mu g fentanyl to an ultra-low (sub-an aesthetic) dose of intrathecal bupivacaine provides adequate anaesthesia for out-patient anorectal surgery, without increasing side-effects or delaying hospital discharge. Patients were randomly allocated to receive 2.5 mg 0.5% bupivacaine plus 25 mu g fentanyl (group BF, n = 18) or 5 mg 0.5% bupivacaine alone (group B, n = 17). There were no significant differences in intra-operative outcomes, but mean recovery and discharge times were significantly shorter in group BF. There were no between-group differences in hypotension, bradycardia or respiratory depression and post-operative complications were comparable, apart from pruritus which was significantly more frequent in group BF. Fewer patients requested analgesic medication in the early post-operative period in group BF than in group B. In conclusion, 25 mu g intrathecal fentanyl added to ultra-low dose (2.5 mg) bupivacaine provided good-quality spinal anaesthesia and reduced post-operative analgesic requirement in patients undergoing ambulatory anorectal surgery.