Background: The approach to unconscious patients in the emergency department (ED) is difficult, often depends on local resources and interests, and workup strategies often lack standardization. One reason for this is that data on causes, management, and outcome of patients who present to the ED with sudden onset unconsciousness of unknown cause is limited. Objectives: This study was performed to analyze the causes of acute impaired consciousness in patients in an interdisciplinary ED. Methods: Here, we analyzed all patients who were admitted to the ED of a tertiary care hospital with the dominating symptom of “sudden onset unconsciousness” within 1 year (September 2014 until August 2015). Patients with a clear diagnosis at arrival that explained the altered state of consciousness or other dominating symptoms at the time of arrival were not included. Results: A total of 212 patients were analyzed. In 88% of the patients, a final diagnosis could be established in the ED. Most common causes for unconsciousness were cerebrovascular diseases (24%), infections (14%), epileptic seizures (12%), psychiatric diseases (8%), metabolic causes (7%), intoxications (7%), transient global amnesia (5%), and cardiovascular causes (4%). The diagnoses were predominantly established by physical examination in combination with computed tomography (23%) and by the results of laboratory testing (25%). In-hospital mortality was 11%, and 59% of all patients were discharged with a Glasgow Outcome Score of 2–4. Conclusions: This analysis demonstrates a large variety of etiologies in patients with unknown unconsciousness of acute onset who are admitted to an ED. As neurological diagnoses are among the most common etiologies, neurological qualification is required in the ED, and availability of diagnostics such as cerebral imaging is indispensable and recommended as an early step in a standardized clinical approach.