 Only validated blood pressure monitors SpaceLabs 90207 and 90217 will be used. Before each ambulatory blood pressure measurement, blood pressure monitors will be set up in a standardized way. A cuff of proper size should be secured to the left arm of the patient. Before the commencement of ambulatory blood pressure measurement, blood pressure will be measured twice using mercury sphygmomanometer. Immediately after these 2 measurements, blood pressure will be measured twice manually with the monitor. Then, blood pressure will be measured automatically according to predefined time intervals of the blood pressure monitor every 20 minutes from 8 am to 10 pm and every 30 minutes from 10 pm to 8 am. At the end of ambulatory blood pressure monitoring, data will be transferred to a computer program. The same size cuff and the same arm should be used for all 3 ambulatory measurements for a patient. On the monitoring day, the patient should record the time to go to bed and to get up, the use of medications, and physical activity. 