A small difference in blood pressure readings between arms is normal. However, a difference of more than 20 mm Hg for systolic pressure (top number) or more than 10 mm Hg for diastolic pressure (bottom number) can be a sign of an underlying narrowing of the main arteries to that arm. A difference of 10 to 15 mm Hg for upper systolic pressure that shows up repeatedly is a risk marker for vascular disease and future heart attacks. Always have your doctor check both arms for blood pressure; whichever is higher, use that arm for future blood pressure readings.