The epidemiology and prognosis of ''fainting'' or syncope has puzzled physicians over the years. Is fainting dangerous? This is a question often asked by the patient--and the answer is ''it depends on a lot of things''. The diverse pathophysiology of syncope and the underlying comorbidites of the patients play an essential role. In epidemiology these factors have major impact on the outcome of the patients. Until recently, even the definition of syncope differed from one study to another which has made literature reviews difficult. Traditionally the data on epidemiology of syncope has been taken from smaller studies from different clinical settings with wide differences in patient morbidity. Through the extensive Danish registries we examined the characteristics and prognosis of the patients hospitalized due to syncope in a nationwide study. The aims of the present thesis were to investigate: 1) the use, validity and accuracy of the ICD-10 diagnosis of syncope R55.9 in the National Patient Registry for the use of this diagnosis in the epidemiology of syncope, 2) diagnostics used and etiology of a random selection of patients who had a discharge diagnosis of R55.9, 3) the incidence, prevalence and cardiovascular factors associated with the risk of syncope, 4) the prognosis in healthy individuals discharged after syncope, and 5) the prognosis of patients after syncope and evaluation of the CHADS2 score as a tool for short- and long-term risk prediction. The first studies of the present thesis demonstrated that the ICD-10 discharge diagnosis could reliably identify a cohort of patients admitted for syncope and that the discharge code carried a high number of unexplained cases despite use of numerous tests. The last studies showed that syncope is a common cause for hospital contact in Denmark and that the risk of syncope is tightly associated with cardiovascular co-morbidities and use of pharmacotherapy. Furthermore in patients with no co-morbidities (or healthy individuals), syncope is a significant and independent prognostic factor of adverse cardiovascular outcome and death compared to the background population. Lastly, evaluation of the CHADS2 score, as a tool for risk stratification, showed that it provided additional prognostic information on short- and long-term cardiovascular mortality in syncope patients compared to controls.