In an academic pediatric Emergency Depratment (ED), we performed a randomized clinical trial of children < 7 years of age, who required i.v. access and who had failed the first i.v. attempt. We randomized patients to either continued standard i.v. attempts or ultrasound-assisted attempts. Clinicians involved in the study received one hour of training in ultrasound localization of peripheral veins. In the ultrasound group, vein localization was performed by an ED physician who marked the skin overlying the target vessel. Intravenous cannulation attempts were then immediately performed by a pediatric ED nurse who relied on the skin mark for vessel location. We allowed for technique cross-over after two failed i.v. attempts. We recorded success rate and location of access attempts.