Objective The purpose of the study was to evaluate the value of ambulatory blood pressure monitoring (ABPM) and plasma catecholamines in the management of orthostatic hypotension. Method: Fifteen patients (four men, 11 women; age, 53.3 ±21.1 years) with orthostatic hypertension were included in the study. Seven had dysautonomia (group 1; multiple systemic atrophy, pure autonomk failure, orthostatic hypotension in elderly people); eight had orthostaric hypotension of other origins (group 2; hypo-volaemia, neurodystonia, vaso-vagal syncope). ABPM and plasma catechohmine assays (high-pressure liquid chromatography) were carried out in all patients in supine and then sanding positions. Results: Systolic/diistolic blood pressure was 131.2±31.9/78.1 ± 12.0mmHg when supine and 112.1 ±25.3/75.4± 15.8 standing (n = 15). The systolic standing-induced decrease in blood pressure after 1 and lOmin (ASBP) correlated with the standard deviation and variation coefficient of mean SBP (r=0.78, P<0.01 and r=0.82, P<0.01 at 1 min; r=0.80, P<0.01 and r=0.81, P<0.01 at 10 min), but not with noradrenaline or adrenaline levek. There was a significant correlation between diastolic nycthemeral blood pressure variability expressed by mean night-time diastolic blood pressure (DBP)/mean daytime DBP ratio (diastolic N/D) and standing-induced decrease in DBP after 1 min (r=0.59, P<0.05). ASBP at 1 and lOmin, ADBP at Imin (P=0.02, P=0.05 and P=0.01, respectively) and systolic and diastolic N/D (P=0.02 and P=0.01) were significantly different in groups 1 and 2. Five out of the seven patients with Anoradrenaline <50pg/ml but only one out of the six patients with Anoradrenaline > 50 pg/ml had dysautonomia (P<0.01). Conclusions: The study demonstrates a relationship between standing-induced blood pressure variation, blood pressure variability and the circadian blood pressure pattern in patients with orthostatic hypotension. Plasma catecholamine assays could be used as an index appropriate for understanding the mechanism of orthostatic hypotension in this population.