Question: what is a dangerously high blood pressure?
Answer: Sheps, M.D. A hypertensive crisis is a severe increase in blood pressure that can lead to a stroke. Extremely high blood pressure — a top number (systolic pressure) of 180 millimeters of mercury (mm Hg) or higher or a bottom number (diastolic pressure) of 120 mm Hg or higher — can damage blood vessels.