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) ≥ 20 mm Hg with simultaneous symptoms; (2) a decrease in SBP between 10 and 20 mm Hg, but a SBP ≤ 90 mm Hg with or without symptoms; and (3) a nonsignificant decrease in SBP in the ED, but a prehospital value ≤ 80 mm Hg was documented that prompted fluid therapy before standardized measurements. Among 788 patients with syncope seen in the ED during a 20-month period, 650 were included in the study, and 579 (89%) had standardized measurements of SBP. According to diagnostic criteria, orthostatic hypotension was considered to be the cause of syncope in 156 of them (24%). Fifty-eight patients (37%) had drug-induced hypotension, 33 (21%) hypovolemia, 19 (12%) postprandial hypotension, and 46 (29%) idiopathic hypotension. Asymptomatic SBP changes ≥ 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. (Am J Emerg Med 2002;20:497-501.