More than two decades ago, the Agency for Healthcare Research and Quality developed its Patient Safety Indicators (PSIs) to monitor potentially preventable and severe adverse events within hospitals. Application of PSIs outside the US was explored more than a decade ago, but it is uncertain whether they remain relevant within Europe, as no up-to-date assessments of overall PSI-associated adverse event rates or interhospital variability can be found in the literature. This article assesses the nationwide occurrence and variability of thirteen adverse events for a case study of Belgium. We studied 4,765,850 patient stays across all 101 hospitals for 2016-18. We established that although adverse event rates were generally low, with an adverse event observed in 0.1 percent of medical hospital stays and in 1.2 percent of surgical hospital stays, they were higher than equivalent US rates and were prone to considerable between-hospital variability. Failure-to-rescue rates, for example, equaled 23 percent, whereas some hospitals exceeded nationwide central line-associated bloodstream infection rates by a factor of 8. Policy makers and hospital managers can prioritize PSIs that have high adverse event rates or large variability, such as failure to rescue or central line-associated bloodstream infections, to improve the quality of care in Belgian hospitals.