Background & aims: Currently, there are no clinical or laboratory parameters that can be used efficiently to predict the prognosis of critically ill patients, but in some situations, raw bioelectrical impedance parameters have been shown to be useful. The purpose of this study was to investigate the behavior of reactance and resistance in the severity of the critically ill pediatric patient. Methods: We prospectively analyzed bioelectrical impedance in a sample of 332 critically ill pediatric patients submitted to mechanical ventilation. The values taken on admission and discharge were correlated with major outcomes to the critically ill patient. Results: We found an association of low values of Xc/H (<27.7 Ohm/m) and of R/H (<563.6 Ohm/m) on admission with multiple organs dysfunction greater or equal to 4. Both R/H and Xc/H increased significantly between admission and discharge among survivors, while among nonsurvivors there was a trend of decrease between admission and the last measurement before death. Conclusions: Bioelectrical impedance is a useful tool for monitoring of critically ill pediatric patients. A possible role of R/H and Xc/H, especially the latter, as a predictive biomarker of evolution for septic shock and organ dysfunction still remains to be elucidated.