Background and objective: In small children, the placement of arterial catheters can be technically challenging for even the most experienced anaesthetist. We investigated whether ultrasound imaging would improve the success rate and reduce time demand and complications of radial artery cannulation. Method: In this prospective randomized study, we performed radial artery cannulation in 30 small children (age 40 ± 33 months) using two different techniques for localization of the vessel. In Group 1 (n = 15), the traditional palpation method was used, while in Group 2 (n = 15) cannulation was directed by vascular ultrasound imaging. In addition, we used ultrasound to determine the cross-sectional area of the radial artery with and without dorsiflexion. For statistical analysis, the non-parametric U-test for non-paired data and the Wilcoxon signed rank sum test for paired data were used. Differences were considered significant, when P < 0.05. Results: Ultrasound-guided puncture was successful in all children of Group 2 compared to only 12 of 15 (80%) children in Group 1. Fewer attempts with the imaging technique were required than with the traditional technique (20 vs. 34, P < 0.05). Dorsiflexion significantly reduced the mean cross-sectional area of the artery by 19%. Conclusion: The current pilot study suggests that ultrasound guidance is appropriate for radial artery catheter insertion, sharing many of the benefits of ultrasound-guided central vein catheter insertion.