A small difference in blood pressure readings between arms is normal. However, a difference of more than 20 mmHg for systolic pressure (top number) or more than 10 mmHg 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 mmHg for upper systolic pressure that shows up repeatedly is a risk marker for vascular disease and future heart attacks.

Always have your doctor check blood pressure in both your arms and use that arm, with higher reading, for future blood pressure readings.