All admissions to a pediatric intensive care unit (PICU) during a period of 33 months were retrospectively evaluated. We used the Clinical Classification System (CCS) at the time of admission; age, diagnosis, mortality and PICU stay were also evaluated. There were 1027 admissions: 794 (77.3%) with CCS-IV, 228 (22.2%) CCS-III and five (0.5%) CCS-II. The global mortality rate was 36.8%, being 46.3, 4.4 and 0% for groups IV, III and II, respectively. Mortality rate was significantly higher in group IV when compared with III (P < 0.001). Average PICU stay was 5.1 days (range: 0.5 hours-75 days), being higher in CCS-IV than in CCS-III (5.8 vs 2.7, P < 0.001). Comparing CCS with age and diagnosis, were found that mortality was significantly higher for any age group and diagnosis for CCS-IV than for III. The CCS allows to see statistical differences in mortality between groups III and IV, as well as PICU stay.