Data from two 3-month time periods before and after the arrival of a pediatric intensivist were collected prospectively and compared to determine the intensivist's impact on ICU mortality, use of monitoring and therapeutic modalities, and efficiency of ICU bed utilization. Severity of illness and care modalities were determined daily for all patients with the Physiologic Stability Index and the Therapeutic Intervention Scoring System. The only major organizational change in the postintensivist period was the organization and implementation of a daytime ICU team. Case mix variables, including sex; medical/surgical, emergency/elective, and diagnostic distributions; and nursing hours/patient day, were equivalent in the pre-intensivist and postintensivist samples. After the intensivist's arrival, there was a significant decrease in admissions with very low severity of illness (Physiologic Stability Index less than 4; 52% vs. 34%; p less than .05) and a significant decrease in bed utilization by patients who received only monitoring services (27% vs. 17% of bed days; p less than .001). The severity of the illness-adjusted ICU mortality rate was significantly higher in the pre-intensivist period than in the postintensivist period (weighted mean mortality difference 5.3 +/- 2.6%; p less than .05). The incidence of both therapeutic and monitoring modalities increased in the postintensivist period. These results indicate that a pediatric intensivist can improve mortality rates and efficiency of bed utilization in the pediatric ICU.