The authors report the economic analysis of their study of adverse drug events (ADEs) at the Massachusetts General Hospital and Brigham and Women’s Hospital in Boston. Using a case-control design, the authors examine the direct costs to the hospital of preventable and nonpreventable ADEs. Although some results did not achieve statistical significance and others did so marginally, the authors demonstrate an increased length of stay and associated hospital charges in the ADE group relative to the controls and in the cases and controls relative to the remainder of the study cohort. While cautioning about the generalizability of their results, the authors estimate a national cost figure of $4 billion (in 1997) for all ADEs and $2 billion for preventable events. This detailed analysis continues to serve as an important reference to motivate policy and institutional investment in systems intended to prevent ADEs.