Hospitalized children are particularly vulnerable to specific types of errors, such as medication errors. This Canadian study used a trigger tool approach to estimate the frequency of all types of adverse events in hospitalized children, and found that nearly 1 in 10 pediatric patients suffers an adverse event while hospitalized. This prevalence is similar to classic studies performed in adult populations. Preventable adverse events, which accounted for approximately half of all events, were particularly common in children undergoing surgery or requiring intensive care. Diagnostic errors also accounted for a significant proportion of preventable adverse events. A preventable error in a critically ill 8-month-old child is discussed in an AHRQ WebM&M commentary.