Blood pressure (BP) recordings often differ between arms, but the extent to which these differences are reproducible and whether the differences have prognostic importance is unknown. We enrolled 421 consecutive patients from a medicine and a renal clinic at a veterans' hospital. Three BP recordings were obtained in each arm using an oscillometric device in a sequential manner and repeated in 1 week. Patients were followed for all-cause mortality <or=7 years. The right arm had 5.1-mm Hg higher systolic BP that attenuated by approximately 2.2 mm Hg a week later. Systolic BP dropped 6.9 mm Hg over 1 week and by an additional 5.3 mm Hg in patients with chronic kidney disease. Accounting for the visit and arm effect improved the reproducibility of the BP measurements. The intraclass correlation coefficient was 0.74, which improved to 0.88 after accounting for visit and 0.93 after accounting for arm. The crude mortality rate was 6.33 per 100 patient-years. Every 10-mm Hg difference in systolic BP between the arms conferred a mortality hazard of 1.24 (95% CI: 1.01 to 1.52) after adjusting for average systolic BP and chronic kidney disease. BP differences between arms are reproducible and carry prognostic information. Patients should have evaluation of BP in both arms at the screening visit.