The most recent guidelines do not mention which arm to use to measure blood pressure or interarm blood pressure differences. In 357 women and 171 men, mean age 79 +/- 10 years, 2 geriatricians simultaneously measured brachial artery blood pressure (BABP) with the patient in the sitting position. All blood pressure measurements were performed using the same 2 machines, which were calibrated and marked 1 and 2. The machines and cuffs were transferred to the opposite arm for a repeat measurement in all patients and the results of the 2 blood pressures averaged. Patients with conditions that may cause a disparity in blood pressure between the right and left arms were not included in this study. The right systolic BABP was >/= 10 mm Hg higher than the left systolic BABP in 35 of 528 patients (7%), and the left systolic BABP was >/= 10 mm Hg higher than the right systolic BABP in 35 of 528 patients (7%) (P = not significant). The right diastolic BABP was >/= 10 mm Hg higher than the left diastolic BABP in 16 of 528 patients (3%), and the left diastolic BABP was >/= 10 mm Hg higher than the right diastolic BABP in 12 of 528 patients (2%) (P = not significant). There was no significant difference in prevalence of hypertension, atherosclerotic vascular disease, diabetes mellitus, or hypercholesterolemia in patients with or without a >/= 10-mm Hg difference in right and left systolic BABP and in right and left diastolic BABP. Interarm differences of >/=10 mm Hg in systolic BABP were found in 14% of elderly patients and of >/= 10 mm Hg in diastolic BABP in 5% of elderly patients. The higher blood pressure should be used for the diagnosis of hypertension, and the blood pressure in that arm used for all follow-up blood pressure-evaluating therapy.