Background. Previous work has demonstrated that the International Classification of Diseases 9th Revision (ICD-9) Based Illness Severity Score (ICISS) methodology developed by Rutledge and Osier can perform well in this role as a severity adjustment tool in trauma patients. The purpose of the present study was to extend this previous work to determine the ability of lCISS to predict outcomes in all types of hospitalized patients. Methods. The ICISS methodology was used to derive predictions of survival, length of hospital stay, and hospital charges in the entire study population. Results. A total of 821,455 hospitalized patients in North Carolina in 1996 had complete data available for analysis. The overall hospital mortality rate was 2. 9%. ICISS was an accurate predictor of hospital survival in all hospitalized patients (accuracy 95.9%, sensitivity 97. 2%, and specificity 52. 7%.) The area of the receiver operator characteristic curve was 0.93. By adding age to the model, the area under the receiver operator characteristic curve increased to 0.95. ICISS also explained a large amount of the variance in hospital stay and charges (R2 = 0.38 and 0.56, respectively, P < .0001). Conclusions. This study extends previous work suggesting that ICISS may be an important improvement over other presently available severity adjustment models. If these findings are confirmed in comparison with other predictive tools, ICISS may find an important place in assessing illness severity.