The epidemiology of adverse events on a population basis has been well studied, but how these data translate to risks for individual patients is not as clear. The likelihood of suffering an adverse event is directly tied to length of hospitalization, and this study sought to evaluate a complementary question: whether patients who are more severely ill at admission are at increased risk of preventable harm. By linking adverse event data from various sources—including Patient Safety Indicators, voluntary error reports, and infection control reports—to clinical databases, the authors were able to show that higher illness severity is associated with an increased risk of adverse events during hospitalization. These findings are supported by the fact that intensive care unit patients have consistently been shown to experience more adverse events. An AHRQ WebM&M commentary discusses a case of a medication error occurring in an acutely ill patient with multiple underlying comorbidities.