The Institute of Medicine's 2000 report, To Err Is Human, garnered headlines for its estimate that up to 98,000 deaths per year occurred due to medical errors in inpatients. This estimate was immediately questioned, as the incidence of adverse events varies widely even among the studies that formed the basis of the IOM report. This systematic review attempts to address this controversy by evaluating eight published studies of medical error incidence in hospitalized patients. The authors conclude that approximately 1 in 10 patients experiences an adverse event, most commonly relating to postsurgical complications or adverse drug events. Nearly half of the events were considered preventable. The potential impact of patient safety interventions is discussed in light of these findings.