OBJECTIVES: Significant muscle wasting develops in critically ill adults, with subsequent worse outcomes. In the pediatric setting, occurrence and effects of muscle wasting are undescribed; this is in part due to a lack of validated, objective methods for assessing muscle wasting. A single measurement of quadriceps femoris thickness has failed to show consistent reproducibility. We hypothesized that averaging repeated measurements could afford good reproducibility to allow for quadriceps femoris thickness decline detection and monitoring. DESIGN: A prospective bedside observational study. SETTING: Two PICUs. PATIENTS: Mechanically ventilated critically ill children were 15 years and younger. INTERVENTIONS: Transverse and longitudinal axis measurements of quadriceps femoris anterior thickness were undertaken using bedside ultrasound. The average of four measurement values was recorded. The location of measurement was marked for consistency within subsequent measurements by the same or another trained operator, to assess intra- and interoperator repeatability and reproducibility of the technique. Where feasible, serial measurements were undertaken until the time of extubation in a group of children with prolonged PICU stay (> 5 d). MEASUREMENTS AND MAIN RESULTS: Seventy-three children were enrolled to assess intra- and interoperator ultrasound reliability. Their median (25-75 interquartile range) age and weight were 30 months (4.5-96) and 10 kg (5-23.5). In the intraoperator repeatability study, mean relative difference in quadriceps femoris muscle thickness was 0.36% ± 2.5% (lower and upper limits of agreement: -4.5/+5.2%). In the interoperator reproducibility study, intraclass correlation coefficient was 0.998. In the 17 children monitored over their PICU stay, quadriceps femoris thickness significantly decreased at day 5 by 9.8% (p = 0.006) and by 13.3% (< 0.001) at the last performed measurement. CONCLUSIONS: Quadriceps femoris thickness decrease, proposed as a surrogate for muscle mass, is an early, frequent, and intense phenomenon in PICU. Quadriceps femoris ultrasonography is a reliable technique to monitor this process and in future could help to guide rehabilitation and nutrition interventions.