Although the Institute of Medicine's estimate of up to 98,000 deaths yearly from preventable adverse events has become part of popular parlance, in truth, the true burden of medical errors remains controversial. This case–control study analyzed a large claims database of more than 24 million patients to identify errors, deaths, and costs, and found that more than 1.5 million preventable adverse events occur in hospitalized patients yearly, resulting in $19.5 billion in excess costs and 2500 excess deaths yearly. The most common preventable errors were pressure ulcers—considered a never event—and health care–associated infections. By comparing the outcomes of patients who experienced an error to patients with similar illnesses who were not harmed, this study was able to estimate costs and mortality directly attributable to errors. Prior research has found that administrative data may underestimate error incidence, so the true number and impact of errors may be higher than that reported here. Nonetheless, this study represents a significant step forward in defining the epidemiology of error in hospitalized patients.