BACKGROUND: One of the main disadvantages of caudal block is the long latent period before a satisfactory blockade is obtained. Many investigators have used various preparations of local anesthetic solutions to improve the speed of onset. This study was performed to evaluate the effectiveness of prewarming of lidocaine HCl for caudal block. METHODS: Fifty healthy young patients (ASA I) were allocated into two groups, A and B. In group A, the local anesthetic solution were injected at room temperature (25 degrees C), while in group B, they were injected at 37oC. All the caudal block were performed using 2% lidocaine HCl 20 ml with fentanyl 100ug and epinephrine 1:200,000. The onset time was defined as the period from completion of injection until the patient first noticed loss of sensation to pin-prick on perianal region. Assessment of sensory loss was made at 15 seconds interval. We have compared the onset of sensory blockade between groups. The duration of analgesia and any significant side effects were also recorded. RESULTS: The onset of sensory blockade was significantly faster in group B (3.5 +/- 0.5 minutes) than group A (6.2 +/- 0.9 minutes). The duration of analgesia were not significantly changed between groups. Side effects of urinary retention, pruritus and nausea were noted between both groups but the difference was not significant. CONCLUSIONS: We have found that the onset time was 44% faster with warm lidocaine-fentanyl mixture (37 degrees C) than with the room temperature (25 degrees C). The improved clinical usefulness was achieved with no increase in side effects. The technique is a safe and effective method to reduce the latency of onset.