Using the Institute for Healthcare Improvement's Global Trigger Tool, this retrospective study analyzed adverse events at a large health care system in Texas. Approximately one-third of patients experienced at least one adverse event during their hospital stay. The vast majority of these incidents were deemed potentially preventable. Surgical and procedural complications accounted for a large portion of adverse events in the hospital. Less than 5% of the hospital-acquired adverse events identified in this study would have been discovered through voluntary reporting or use of AHRQ Patient Safety Indicators, illustrating the challenges of detecting safety hazards. A previous AHRQ PSNet interview with Dr. David Classen explored the use of trigger tools to measure patient safety.