Low body mass index (BMI) z score is commonly used to define undernutrition, but faltering growth allows for a complementary dynamic assessment of nutritional status. We studied the prevalence of undernutrition and faltering growth at admission in the pediatric intensive care (PICU) setting and their impacts on outcome. All (685) consecutive children (aged 0 to 18 years old) admitted in a single-center PICU over a 1-year period were prospectively enrolled. Nutritional status assessment was based on anthropometric measurements performed at admission and collected from medical files. Undernutrition was considered when z score BMI for age was < ? 2SD. Faltering growth was considered when the weight for age curve presented a deceleration of > ? 1 z score in the previous 3 months. Undernutrition was diagnosed in 13% of children enrolled, and faltering growth in 13.7% mostly in children with a normal BMI. Faltering growth was significantly associated with a history of underlying chronic disease, and independently with extended length of PICU stay in a multivariate analysis. Conclusion: Assessment of nutritional status in critically ill children should include both undernutrition and faltering growth. This study highlights that faltering growth is independently associated with suboptimal outcome in PICU. What is Known: ? Malnutrition, defined according to BMI-for-age z score, is correlated with poor outcome in the critically ill child. ? In this setting, nutritional assessment should consist not only of a static assessment based on BMI-for-age z score but also of a dynamic assessment to identify recent faltering growth. What is New: ? Critically ill children frequently present with faltering growth at admission. ? Faltering growth is a newly identified independent associated factor of suboptimal outcome in this setting (extended length of stay).