Both the systolic and diastolic pressure are important. High blood pressure the causes of which are not well understood is defined, for adults, as a systolic pressure of 140 mm Hg or greater and/or a diastolic pressure of 90 mm Hg or greater. Normal blood pressure is defined as a systolic reading of less than 120 mm Hg and a diastolic reading of less than 80 mm Hg.