BACKGROUND: An inter-arm difference in blood pressure (IADBP) is characteristic of acute aortic dissection (AAD), but which arm shows lower blood pressure (BP) and the mechanism of IADBP has not been fully elucidatedMethods: We identified consecutive patients with chest and/or back pain and suspected acute cardiovascular disease whose BP had been measured in both arms. We retrospectively compared the characteristics of such patients with AAD (n=93) to those without (non-AAD group, n=122). Additionally, we separately compared patients with type A AAD (TAAD group, n=58) or type B AAD (TBAD group, n=35) to non-AAD group. Characteristics included in these comparisons were patients' backgrounds and IADBP-related factors such as systolic BP (SBP) in the right arm (R) and left arm (L), R-L or L-R as the IADBP. Computed tomography (CT) findings of AD extending to the brachiocephalic artery (BCA) and/or left subclavian artery (LSCA) were examined in patients having IADBP. RESULTS: In the TAAD group, the prevalence of R<130mmHg (38%-vs.-19%, p=0.009), L-R>15mmHg (19%-vs.-8%, p=0.047), L-R>20mmHg (14%-vs.-4%, p=0.029) was higher than in the non-AAD group. Multivariate analysis showed L-R>15mmHg with R<130mmHg was independently associated with TAAD (OR 25.97, 95% CI 2.45-275.67, p=0.007). However, IADBP-related factors were not associated with TBAD. AAD patients with L-R>20mmHg were all TAAD, and all aortic dissection extended to BCA just before the right common carotid artery on CT. CONCLUSIONS: IADBP was characterized by R<L with low R in TAAD, but was not associated with TBAD.