Systemic high blood pressure is a very frequent disease. One characteristic is its daily continuous variability. Ambulatory monitoring of blood pressure has some advantages over the clinical blood pressure reading at the doctor's office. It is more precise, reliable, records the pressure at night and during sleep, avoids "white coat" phenomenon, among other things. This does not mean that blood pressure should not be taken during the regular medical visits. If there is any doubt of the presence of hypertension, blood pressure should be taken 3-4 times par day during several days to ensure the diagnosis, because one high blood pressure reading does not establish the diagnosis of systemic hypertension.