We describe the incidence and types of medical errors in emergency departments (EDs) and assess the validity of a survey instrument that identifies systems factors contributing to errors in EDs. We conducted the National Emergency Department Safety Study in 62 urban EDs across 20 US states. We reviewed 9,821 medical records of ED patients with one of 3 conditions (myocardial infarction, asthma exacerbation, and joint dislocation) to evaluate medical errors. We also obtained surveys from 3,562 staff randomly selected from each ED; survey data were used to calculate average safety climate scores for each ED. We identified 402 adverse events (incidence rate 4.1 per 100 patient visits; 95% confidence interval [CI] 3.7 to 4.5) and 532 near misses (incidence rate 5.4 per 100 patient visits; 95% CI 5.0 to 5.9). We judged 37% of the adverse events, and all of the near misses, to be preventable (errors); 33% of the near misses were intercepted. In multivariable models, better ED safety climate was not associated with fewer preventable adverse events (incidence rate ratio per 0.2-point increase in ED safety score 0.82; 95% CI 0.57 to 1.16) but was associated with more intercepted near misses (incidence rate ratio 1.79; 95% CI 1.06 to 3.03). We found no association between safety climate and violations of national treatment guidelines. Among the 3 ED conditions studied, medical errors are relatively common, and one third of adverse events are preventable. Improved ED safety climate may increase the likelihood that near misses are intercepted.