The difference in systolic or diastolic pressure between the left and right hand does not normally exceed 10 mmHg. If the difference in pressure between the left and right hand exceeds 20 mmHg, it is abnormal. The main reason for a large pressure difference between the left and right hand is because of stenosis or occlusion of the subclavian artery, brachial artery or the trunk artery of the head and arm of the clinic. The common cause of arterial stenosis or occlusion is atherosclerosis, which is commonly seen in elderly patients with hypertension and diabetes, but also in young patients with aortitis. When arterial stenosis or occlusion occurs, arterial vascular ultrasound should be done to further clarify, and if necessary, angiography should be done to clarify the diagnosis, and usually when measuring hypertensive patients for the first time, the left and right hand blood pressure should be measured at the same time.