OBJECTIVES: Perform a needs assessment by evaluating accuracy of PICU provider bedside ultrasound measurement of femoral vein diameter prior to utilization of the catheter-to-vein ratio for central venous catheter size selection. DESIGN: Prospective observational cohort study. SETTING: PICU within a quaternary care children's hospital. PATIENTS: PICU patients greater than 30 days and less than 6 years without a femoral central venous catheter. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Gold-standard femoral vein diameter measurements were made by a radiologist, sonographer, or bedside ultrasound expert. PICU providers then repeated the femoral vein diameter measurements, and results were compared by Bland-Altman analysis with a priori accuracy goal of limits of agreement ± 15%. Among recruited patients (n = 27), the median age was 1.1 years (interquartile range 0.5-2.3 yr), weight was 9.0 kg (interquartile range 7.0-11.5 kg), and reference femoral vein diameter was 0.36 cm (interquartile range 0.28-0.45 cm). Providers performed 148 femoral vein diameter measurements and did not meet goal accuracy when compared with the reference measurement with a bias of 4% (95% of limits of agreement -62% to 70%). A majority of patients would have a catheter-to-vein ratio greater than 0.5 using either age-based central venous catheter size selection criterion (14/27) or the provider bedside ultrasound femoral vein diameter measurement (18/27). CONCLUSIONS: PICU provider measurement of femoral vein diameter by bedside ultrasound is inaccurate when compared with expert reference measurement. Central venous catheter size selection based on age or PICU provider femoral vein diameter measurement can lead to a catheter-to-vein ratio greater than 0.5 and potentially increase the risk of catheter-associated venous thromboembolism. Structured bedside ultrasound training with assessment of accuracy is necessary prior to implementation of venous thromboembolism reduction efforts based on catheter-to-vein ratio recommendations.