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 your arms for blood pressure and which of the two is higher, use that arm for future blood pressure readings.