Using ultrasound imaging, the optimal angle for needle insertion during caudal epidural injection in children was estimated. After general anaesthesia, ultrasonography was performed at the sacral hiatus in 130 children aged 2-84 months positioned in the lateral position. The median [range] values for the intercornual, caudal space depth and the distance from skin to the posterior sacral bony surface were 17.0 [9.6-24] mm, 3.5 [1-8] mm and 21.0 [10-39] mm, respectively. The optimal angle showed no significant correlation with age, weight, height or body surface area. The median [range] calculated optimal angle for the needle was 21.0 [10-38] degrees. We conclude that the needle should be inserted at about 20 degrees to the skin to avoid puncture of the bone and potential intra-osseous injection.