We sought to determine the prevalence of orthostatic hypotension as a cause of syncope in the emergency setting, and describe the characteristics of patients diagnosed with this condition. Blood pressure orthostatic changes were measured prospectively in a standardized fashion up to 10 minutes, or until symptoms occurred, in all consecutive patients with syncope as a chief complaint presenting in the emergency department (ED) of a primary and tertiary care hospital. Patients unable to stand-up were excluded. Hypotension was considered to be the cause of syncope when there was: (1) a decrease in systolic blood pressure (SBP) >or= 20 mm Hg with simultaneous symptoms; (2) a decrease in SBP between 10 and 20 mm Hg, but a SBP or= 20 mm Hg were found in 10% of patients with syncope attributed to other causes. Compared with patients with vasovagal disorder, those with orthostatic hypotension were older; had more comorbid conditions including hypertension, organic heart disease, and abnormal electrocardiogram; were taking more hypotensive medications; and required more frequently hospitalization (P <.01). We concluded that standardized blood pressure measurement in the ED enabled to strongly implicate orthostatic hypotension as a cause of syncope in 24% of patients with this symptom. Drug-related hypotension was the most frequent cause for this disorder.