Physiological stability index (PSI) is a score proposed to evaluate the severity of different diseases in pediatric intensive care units (PICU) by giving scores to the deterioration of 34 measurable physiological variables. In order to validate PSI, it was prospectively applied to 132 patients who entered our PICU. A significant difference (p less than 0.001) was found between the average score of those patients who survived (means: 6.1) versus those who did not (means: 21); No difference was found between theoretical mortality rates obtained by logistic regression analysis and those obtained according to PSI score ranks even thought direct hemodynamic measurements like cardiac output and capillary (wedge) pulmonary pressure were not considered in this study. In conclusion, the PSI is a useful method to measure the severity of illness in pediatric intensive care and to compare results among PICU.