The "white coat effect" has been investigated by non-invasive automatic blood pressure monitoring in patients with hypertension, defined by casual blood pressure readings. A significant "white coat effect" has been demonstrated in 30 (32%) of the 93 patients: the average values were 17/9 mmHg and 6 beat/min, the highest values were 37/29 mmHg and 13 beat/min. The examination has been repeated after 24 hours in 11 cases and the phenomenon was reproducible. The "white coat effect" did not disappear even when the changes were compared to the averages of three subsequent automatic blood pressure measurements. There were significantly more women, than men among the "white coat" positive patients. However, no difference was found in age, occupation and the known duration of hypertension. Neither was any correlation between the "white coat effect" and the blood pressure reaction to mental arithmetic test. It is emphasized that the casual readings can significantly overestimate the blood pressure. This finding must be considered especially in the diagnosis of borderline hypertension.