SummaryWe developed a physiology-based scoring system, the Physiologic Stability Index (PSI) to assess severity of acute illness in the total population of pediatric Intensive Care Unit (ICU) patients. Thirty-four variables from seven physiologic systems were chosen, and the degree of abnormality of each variable was assigned a score reflecting the clinical importance of the derangements. Validity was demonstrated by comparing PSI to hospital mortality and to two other methods that reflect severity of illness, the Clinical Classification System (CCS) and the Therapeutic Intervention Scoring System (TISS). Four hundred and twenty three consecutive admissions to a multidisciplinary ICU were followed daily. Patients classified into higher CCS classes had significantly higher PSI scores (P<0.001), and there was a highly significant correlation (P<0.001) between PSI and TISS scores. The linear-logistic regression of observed mortality versus PSI was highly significant (P<0.0001) and provided an excellent fit. Highly significant differences between survivors and nonsurvivors were observed for PSI scores (P<0.001), as well as for composite slopes of the regression of PSI scores versus days of care (P<0.001). These data demonstrate validity of the PSI scoring system.