BACKGROUND: Early ambulation after anorectal surgery may be possible by relieving pain with intrathecal morphine and decreasing headache and backache with a much thinner and pencil-point needle. The difference in urinary peak flow rate in upright posture compared with recumbent position was reported to be highly significant, although acute urinary retention induced by intrathecal morphine may be decreased by early ambulation. METHODS: Eighty patients due for anorectal surgery were selected to receive spinal anesthesia. Subjects in group A (n=40) received 0.5% tetracaine 5 6 mg through a 25-gauge Quinke needle while group B (n=40) received 0.5% tetracaine 5 6 mg and intrathecal morphine 0.2 mg through a 27-gauge Whitacre needle. Postoperatively, group A received 24 hours bed rest and group B was recommended to walk as soon as possible. The duration of pain relief, onset time to ambulation, headache, backache, urinary retention and nausea were observed in both groups. RESULTS: The average onset time of early ambulation and duration of postoperative pain relief in group B was 3.6 +/- 1.0 and 15.1 +/- 3.5 hours respectively. The incidence of postspinal headache and backache was 2.5 and 5.0% in group B respectively and decreased significantly in comparison with group A (20.0 and 22.5%) respectively (P < 0.05). However, the incidence of postoperative nausea was 35.0% in group B and increased in comparison with group A (12.5%) (P < 0.05). Group B yielded a relatively lower urinary retention rate (40.0%) than group A (52.5%), but the difference did not reach statistical significance. CONCLUSIONS: Intrathecal morphine infused by a 27-gauge Whitacre needle provided postoperative pain relief with early ambulation, and decreased headache and backache, but we suggest that there is a need to select another drug or method instead of intrathecal morphine to decrease the incidence of urinary retention and nausea.