Background: Resident physicians are frontline providers with a unique vantage point from which to comment on patient safety-related events., Methods: We surveyed trainees at 2 teaching hospitals about experiences with adverse events (AEs), mistakes, and near misses, as well as the potential causes., Results: Responses were obtained from 821 (57%) of 1440 eligible trainees. Analysis was restricted to 689 clinical trainees. More than half (55%) reported ever caring for a patient who had an AE. The most common types of AEs were procedural and medication related. More than two thirds of AEs were considered significant. Of the most recent AEs, 24% were attributed to mistakes. The most common reasons for mistakes, as perceived by residents, were excessive work hours (19%), inadequate supervision (20%), and problems with handoffs (15%). In the last week, 114 respondents (18%) reported having a patient with an AE; of these, 42 (37%) reported AEs involving a mistake for which they considered themselves responsible. In addition, 141 (23%) reported near-miss incidents in the last week for which they considered themselves responsible. In multivariate analyses, significant predictors of AEs in the last week were inpatient rotation, duty hours in the last week, and procedural specialty. Predictors of near-miss errors in the last week were inpatient rotation, days of fatigue in the last month, and postgraduate year 1 status., Conclusions: These findings support the perception that AEs are commonly encountered by physicians and often associated with errors. Causes of errors in teaching hospitals appear to be multifactorial, and a variety of measures are necessary to improve safety. Eliciting residents' perspectives is important because residents may perceive events, actions, and causal relationships that medical record reviewers or observers cannot.