Abstract. Objective: To define a quality assurance instrument to evaluate errors in diagnostic processes made by physicians in the emergency department (ED). Methods: This was a retrospective clinical investigation of inpatient ED records. Over a six-year period, 5,000 medical records of admitted patients were randomly selected for evaluation. Each record was initially examined by one of five physician evaluators. If the primary ED diagnosis differed from the primary discharge diagnosis, the ED record was inspected to determine reasons for the misdiagnosis. The authors considered several aspects of the diagnostic process, including patient history, tests ordered, interpretation of clinical data, choice and performance of procedures, injury pattern recognition, reasoning, and evaluation. Records that demonstrated errors in the diagnostic process were reevaluated for the same diagnostic process errors by a sixth physician. Disagreements regarding suspected errors in the diagnostic process were settled by discussion. Finally, to determine potential medical consequences of the misdiagnosis, one individual reviewed the complete medical records of patients whose ED medical records were scored with errors by both evaluators. Interevaluator reliability was assessed using Cochran's Q-test with a selected series of medical records. Results: Twenty-eight records (0.6%) were found to contain one or more errors in the diagnostic process that contributed to misdiagnosis. For these patients appropriate diagnosis was not made until one to 16 days after admission. Three patients of 18 whose records were available for detailed review may have suffered complications that resulted, in part, from the delay in diagnosis and subsequent treatment. Significant interevaluator reliability for identification of errors in the diagnostic process was obtained (p > 0.1). Conclusions: A two-tiered evaluation of ED records selected by inconsistent initial and final diagnoses can be used reliably to screen for errors in the diagnostic process made by emergency physicians (EPs). The rate of physician error contributing to a misdiagnosis is very low, suggesting that EPs are delivering quality patient care.