Background: Caudal block with the use of an adequate dose of bupivacaine, and combined with a general anesthesia (GA) provides intra-operative anesthesia and postoperative analgesia. No study has examined the use of 3 mg/Kg bupivacaine for intra-abdominal surgery in pediatric patients in clinical practice.Objective: Compare the effectiveness of three mg/Kg bupivacaine administered as 1.2 mL/Kg 0.25% bupivacaine and 1.5 mL/Kg 0.2% bupivacaine for caudal block in pediatric patients undergoing intra-abdominal surgery.Methods: In a randomized, double-blinded clinical trial, patients (age: 6 months -7 years) were randomly assigned into one of two groups (n= 40) to receive a caudal block with either 1.2 mL/Kg 0.25% bupivacaine (group A) or 1.5 mL/Kg 0.2% bupivacaine (group B), with morphine 50 μg/Kg. The effectiveness of intra-operative anesthesia, complications, and requirements for post-operative analgesia were evaluated.Results: Data were available for 74 pediatric patients. There were no significant differences between the two groups in baseline characteristics. Intra-operatively, the numbers of patients who required a rescue analgesic were comparable between the groups (67% in group A and 63% in group B). The numbers of patients who required a muscle relaxant were also comparable between groups (49% in group A and 57% in group B). The time from discontinuation of the volatile anesthetic to extubation was significantly shorter in group B (9.5±1.1 minutes) than group A (14.3±0.9 minutes), p < 0.01. The time from initial caudal block to the first analgesic required in the recovery room was significantly longer in group B (202±45 minutes) than in group A (149±27 minutes). The time from the caudal block to the first analgesic required in the ward was significantly longer in group B (10.4±3.1 hours) than in group A (8.2±2.0 hours). Overall fentanyl requirements were comparable between groups, 52.5±2.0 μg in group A and 49.5±3.0 μg in group B.Conclusion: Caudal block by either 1.2 mL/Kg 0.25% bupivacaine plus morphine 50 μg/Kg or 1.5 mL/Kg 0.2% bupivacaine plus morphine 50 μg/Kg provided effectively equivalent intra-operative analgesia and surgical relaxation. However, a caudal block with 1.5 mL/Kg 0.2% bupivacaine plus morphine 50 μg/Kg provided superior prolonged analgesic advantages compared with 1.2 mL/Kg 0.25% bupivacaine plus morphine 50 μg/Kg in pediatric patients undergoing intra-abdominal surgery.