Study objectives: Diagnostic errors in the emergency department (ED) are an important patient safety concern, but little is known about their cause. We identify types and causes of missed or delayed diagnoses in the ED. Methods: This is a review of 122 closed malpractice claims from 4 liability insurers in which patients had alleged a missed or delayed diagnosis in the ED. Trained physician reviewers examined the litigation files and the associated medical records to determine whether an adverse outcome because of a missed diagnosis had occurred, what breakdowns were involved in the missed diagnosis, and what factors contributed to it. Main outcome measures were missed diagnoses, process breakdowns, and contributing factors. Results: A total of 79 claims (65%) involved missed ED diagnoses that harmed patients. Forty-eight percent of these missed diagnoses were associated with serious harm, and 39% resulted in death. The leading breakdowns in the diagnostic process were failure to order an appropriate diagnostic test (58% of errors), failure to perform an adequate medical history or physical examination (42%), incorrect interpretation of a diagnostic test (37%), and failure to order an appropriate consultation (33%). The leading contributing factors to the missed diagnoses were cognitive factors (96%), patient-related factors (34%), lack of appropriate supervision (30%), inadequate handoffs (24%), and excessive workload (23%). The median numbers of process breakdowns and contributing factors per missed diagnosis were 2 and 3, respectively. Conclusion: Missed diagnoses in the ED have a complex cause. They are typically the result of multiple breakdowns in the diagnostic process and several contributing factors.