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Background Ketamine is used increasingly in paediatric anaesthetic practice to prolong the action of a caudal block. This study was designed to determine if adding S(+)-ketamine 0.5 mg kg −1 allows a lower concentration of levobupivacaine to be used for caudal anaesthesia without loss of clinical effectiveness. Methods One hundred and sixty-four children (ASA I or II) aged 3 months–6 yr were randomly allocated to receive 1 ml kg −1 of levobupivacaine 0.15% with 0.5 mg kg −1 S(+)-ketamine (Group 1), levobupivacaine 0.175% with 0.5 mg kg −1 S(+)-ketamine (Group 2), or levobupivacaine 0.2% (Group 3) by the caudal route. Pain, motor block, sedation, and requirement for postoperative analgesia were assessed up to 6 h after operation. Results There was no significant difference between the groups in effectiveness at first surgical incision. Significantly lower analgesic requirements were reported in Group 2 compared with Group 3 at wakeup, 180 and 360 min after operation. Time to first rescue analgesia was longer in Group 2 compared with Group 1 or 3. Kaplan–Meier survival analysis of analgesia free time demonstrated a significant advantage of Group 2 over Groups 1 and 3 (log rank P =0.05). The incidence of postoperative motor block was not significantly different between the groups. No excess sedation or dysphoric reactions were observed in the ketamine groups. Conclusions The addition of 0.5 mg kg −1 S(+)-ketamine to levobupivacaine 0.175% for caudal analgesia for lower abdominal and urological surgery is significantly more effective in providing postoperative analgesia than levobupivacaine 0.15% with 0.5 mg kg −1 S(+)-ketamine or levobupivacaine 0.2%.