Backgrounds. We examined the effect on the prognosis of critically ill pediatric patients after a closed ICU policy was implemented into an adult ICU that admitted children. Materials and Methods. We assessed the Pediatric Index of Mortality 2 (PIM2) score of pediatric patients (≤15 y.o.) admitted to the ICU from 2001 to 2009. In our teaching hospital, the department for intensive care was established in January 2004. Since then, for critical care patients, we have followed a closed ICU policy with full-time intensivists. We subsequently compared PIM2 scores and the ratio of observed-to-expected deaths (O/E ratio) for three three-year periods: 2001–2003 (before closed policy), 2004–2006, and 2007–2009. Results. Data was collected from 532 pediatric patients. While the PIM2 score statistically significantly increased from 0.066±0.130 for 2001–2003 to 0.114±0.239 for 2004–2006 and 0.086±0.147 for 2007–2009, the O/E ratio decreased from 1.49 for 2001–2003 to 0.82 for 2004–2006 and remained at 0.82 for 2007–2009. Conclusion. The O/E ratio for critically ill pediatric patients improved after the establishment of a closed policy in an adult ICU that admitted children.