Objectives: Sedation-related adverse events in critically ill pediatric patients lack reproducible operational definitions and reference standards. Understanding these adverse events is essential to improving the quality of patient care and for developing prevention strategies in critically ill children. The purpose of this study was to test operational definitions and estimate the rate and site-to-site heterogeneity of sedation-related adverse events. Design: Prospective cohort study. Setting: Twenty-two pediatric intensive care units in the United States enrolling baseline patients into a prerandomization phase of a multicenter trial on sedation management. Patients: Pediatric patients intubated and mechanically ventilated for acute respiratory failure. Data Extraction: Analysis of adverse event data using consistent operational definitions from a Web-based data management system. Measurements and Main Results: There were 594 sedation-related adverse events reported in 308 subjects, for a rate of 1.9 adverse events per subject and 16.6 adverse events per 100 pediatric intensive care unit days. Fifty-four percent of subjects had at least one adverse event. Seven (1%) adverse events were classified as severe, 347 (58%) as moderate, and 240 (40%) as mild. Agitation (30% of subjects, 41% of events) and pain (27% of subjects, 29% of events) were the most frequently reported events. Eight percent of subjects (n = 24) experienced 54 episodes of clinically significant iatrogenic withdrawal. Unplanned endotracheal tube extubation occurred at a rate of 0.82 per 100 ventilator days, and 32 subjects experienced postextubation stridor. Adverse events with moderate intraclass correlation coefficients included: Inadequate sedation management (intraclass correlation coefficient = 0.130), clinically significant iatrogenic withdrawal (intraclass correlation coefficient = 0.088), inadequate pain management (intraclass correlation coefficient = 0.080), and postextubation stridor (intraclass correlation coefficient = 0.078). Conclusions: Operational definitions for sedation-related adverse events were consistently applied across multiple pediatric intensive care units. Adverse event rates were different from what has been previously reported in single-center studies. Many adverse events have moderate intraclass correlation coefficients, signaling site-to-site heterogeneity.