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Background: Caudo-lumbar and caudo-thoracic epidural anesthesia is an established technique that carries a low risk of dural puncture or spinal cord trauma in infants. Traditionally catheter advancement is based on external measurements. However, malpositioning of catheters are known to occur. We hypothesized that caudal-epidural catheters inserted under real-time ultrasound guidance may be more accurate than the accuracy of the measurements traditionally used for their placement. Methods: We studied 25 patients, aged 2 days to 5 months, posted for abdominal or thoracic surgery, receiving general anesthesia followed by caudo-epidural continuous block. External measurement defined as the distance from the caudal space and the surgically congruent vertebral level was measured in centimeters with the back gently flexed. Subsequently, a caudo-epidural block was performed in the same position. The epidural catheter insertion was followed under real-time ultrasound guidance till the predetermined vertebral level was reached. The actual length placed under real-time ultrasound scan was defined as the actual length. The high-frequency probe was placed longitudinal and paramedian to the spine. The vertebral level was determined by identifying the lumbosacral junction in longitudinal saggital view and counting the vertebrae up from L5. The catheter length at the skin was compared with the length recorded by external measurement. Results: The actual length placed under real-time ultrasound scan were consistently longer than the external distance between the caudal space and selected vertebral level. The mean values of ultrasound were higher than the mean values of external measurement with a difference of 4.28 cm. Accuracy was not affected by age or affected by the selected vertebral level in the age group we studied. Conclusion: We conclude that catheters placed under ultrasound guidance are more accurate than the traditional method developed before the advent of ultrasound in young infants. [ABSTRACT FROM AUTHOR]