Caudal anaesthesia is indicated for surgical procedures lasting less than 90 min. Fentanyl and clonidine are known to prolong postoperative caudal analgesia, but there are no data on their effect on duration of surgical analgesia. We evaluated if the addition of clonidine or fentanyl to local anaesthetics prolonged the duration of surgical analgesia after single shot caudal block in children in a randomized, double-blind study. We studied 64 children, aged 6-108 months, undergoing bilateral correction of vesicoureteral reflux which was expected to last more than 90 min. Patients were allocated to one of four groups: group O received 1 ml kg-1 of a mixture of 0.25% bupivacaine with epinephrine and 1% lidocaine in equal parts; group F received the same mixture of local anaesthetics in addition to fentanyl 1 microgram kg-1; group C received the same mixture of local anaesthetics in addition to clonidine 1.5 micrograms kg-1; and group C + F received the same mixture of local anaesthetics in addition to fentanyl 0.5 microgram kg-1 and clonidine 0.75 microgram kg-1. Single shot caudal block was sufficient in only 57% of children in group O compared with 93% in groups C and F and 86% in group C + F (P = 0.035). Global assessment of anaesthesia, defined as the time from caudal injection to the first administration of analgesic (either during or after surgery), was significantly longer in the three groups of children who received additives compared with local anaesthetics alone (P = 0.035), but there were no differences between the three additive groups. Vomiting was observed only in children who received fentanyl. Addition of clonidine or fentanyl to local anaesthetics prolonged the duration of surgical analgesia of caudal block, allowing single shot caudal anaesthesia to be recommended for surgery lasting 90-150 minutes. Clonidine had some advantages over fentanyl as it did not produce clinically significant side effects.