Heated debates and controversies surrounding the regulation of medical residents' work hours have raged for over twenty years. In the wake of Libby Zion's untimely death in 1984 and resulting recommendations by the Bell Commission, New York State enacted legislation (Code 405.4) governing residents' working conditions and supervision [1]. Since then, there has been growing interest in regulating residents' work hours, culminating recently (2003–2004) in national guidelines and legislation on duty-hour restrictions both in the United States and Europe [2,3]. These regulations continue to be the subject of intense debate and bitter controversy, yet at the same time, the medical profession has become increasingly aware of the complexities of balancing restricted work hours with resident education, well-being, and the profession's key priority—quality of care [4,5]. However, there is still a scarcity of research on the relationship between residents' work schedules and adverse events (AEs) [4,6], defined as injuries due to medical management rather than the underlying condition of the patient [7]. This lack of research is in striking contrast to the wealth of research on the relationship between work schedules and adverse events in industrial and transportation settings [8] (long work hours, for example, were implicated in the catastrophic Exxon Valdez oil spill [9]). In 2000, the Institute of Medicine reported medical errors to be a leading cause of death in the US [10], responsible for 44,000–98,000 in-hospital deaths [7,10,11]. To date, epidemiologic studies, including those upon which the Institute of Medicine's extrapolations were based, have focused on estimating the incidence of adverse events and their consequences—namely, the magnitude of harm resulting from AEs in terms of morbidity (such as patient disability) and mortality [7,10,11]. A benchmark study of New York State hospitals found that 70.5% of AEs produced short-term disability in patients, 2.6% led to permanently disabling injuries, and 13.6% resulted in death [7]. Although the relationships of adverse events to specific provider types/locations and to negligence have been examined [7,11]—with 27.6% of AEs attributed to negligence [7]—the epidemiology of AEs has not been fully clarified. A new study in PLoS Medicine, by Barger and colleagues [12], investigates the contribution of work-hour organization—specifically interns' extended shifts—to adverse events. This relationship is of interest because it could inform future guidelines on residents' working schedules; it could also help hospitals to adopt new strategies for complying with current legislation on restricting work hours and to weigh the costs and benefits of such strategies.