A total of 294 Clinical Classification System (CCS) Classes III and IV patients in a pediatric ICU (PICU) were evaluated in terms of severity of illness and quantity of care. The group was comprised of patients from 3 services: medicine, cardiovascular surgery, and other surgery. Severity of illness was measured by the Physiologic Stability Index (PSI) and quantity of care was measured by the Therapeutic Intervention Scoring System (TISS). Comparisons were made between survivors and nonsurvivors and among the 3 services. Nonsurvivors had significantly higher (p less than .01) PSI and TISS scores than survivors. Medical patients had the highest PSI scores while cardiovascular surgery patients had the highest TISS scores. Analysis of 7-day regression slopes for all survivor groups and medicine and other surgery nonsurvivor groups demonstrated slopes consistent with the expected clinical course. Cardiovascular surgery nonsurvivor slopes were unique and demonstrated increasing stability with stable amounts of care. The PSI/TISS ratio was used to relate levels of physiologic instability to the amount of therapy. Medical patients had the highest ratios and cardiovascular surgery patients had the lowest ratios. Comparisons of survivors and nonsurvivors for the PSI/TISS ratios and regression slopes demonstrated differences that were not evident through comparison of PSI and TISS scores alone.