Background: Continuous caudal epidural block is a useful method in postoperative pain control after perianal surgery. But caudal epidural block has the potential of developing adverse effects such as urinary retention. The goal of this study is to evaluate the analgesic and adverse effect of bupivacaine with fentanyl through continuous caudal epidural block in relation to the concentration of bupivacaine. Methods: We divided the patients randomly into two groups. For group I(n=25) postoperative pain was controlled by continuous caudal epidural infusion at the rate of 4 ml/hr of 0.0625% bupivacaine with 3 ${mu}g$/ml fentanyl: group II(n=14), 0.125% bupivacaine with 3 ${mu}g$/ml fentanyl, respectively, for duration of 48 hours via epidural catheter. We evaluated pain scores with visual analogue scales at 30 mins, 6 hrs, 12 hrs, 24 hrs and 48 hrs after the operation and the incidence of adverse effect, especially urinary retention, for each group. Results: There were no significant differences in the pain score between group I and II. Urinary retention developed in 9 patients(36%) of group I, and 11 patients (78.6%) of group II. Other adverse effects such as pruritus, nausea, vomiting and respiratory depression developed in few patients. Conclusions: While performing continuous caudal epidural block with mixture of bupivacaine and fentanyl after the perianal surgery, we conclude 0.0625% bupivacaine solution is preferable to 0.125% bupivacaine solution because 0.0625% solution resulted in satisfactory analgesia with minimal incidence of adverse effect.