Despite numerous studies over the past three decades, one fundamental patient safety question remains controversial: what proportion of hospitalized patients are harmed by medical care? Prior estimates range from approximately 3% to nearly 17%, but this study found that nearly one-third of patients experienced an adverse event during hospitalization. This study used the Institute for Healthcare Improvement's Global Trigger Tool to detect adverse events and also found that this trigger tool identified significantly more adverse events than voluntary reporting or the AHRQ Patient Safety Indicators. An important caveat is that this study did not assess whether the adverse events detected were preventable. Nevertheless, the results do raise the concern that adverse events remain common despite enhanced safety efforts. The challenges of accurately measuring patient safety events were discussed in an AHRQ WebM&M perspective.