['Radial augmentation index (rAI), a marker of aortic wave reflection, is usually lower in patients with diabetes (DM) than in non-DM subjects, even though atherosclerotic change is advanced in DM.', 'We sought to explore why rAI in DM is lower than in non-DM.', 'We performed radial applanation tonometry in 1787 subjects who had at least one cardiovascular risk factor. The rAI was defined as [late systolic shoulder pressure amplitude (PP2)]/[radial pulse pressure (rPP)]. The late systolic shoulder blood pressure (SBP2) and PP2 of a radial pressure wave were used as estimates of the central SBP and PP (cPP), respectively.', 'The age (65.8 ± 9.8 vs. 65.8 ± 12.1 yrs) and mean brachial SBP (141 ± 16 vs. 141 ± 17 mmHg) were similar between the DM and non-DM groups. The rAI was significantly lower in the DM group (83.3 ± 14.1 vs. 87.3 ± 15.7%, p < 0.001), but clinic PP (62 ± 14 vs. 59 ± 14 mmHg, p < 0.001) and cPP (51 ± 15 vs. 49 ± 15 mmHg, p = 0.019) were significantly greater in the DM group than in the non-DM group. In multivariable analyses adjusting for covariates, the significant determinants of rAI were the estimated glomerular filtration rate (eGFR) (β = 0.17, p < 0.001) in the DM group, and the log-transformed homeostatic model assessment of insulin resistance (HOMA-IR) (β = -0.15, p < 0.001) in the non-DM group. The same trends were also seen for central SBP and cPP.'] Is why the radial augmentation index low in patients with diabetes : The J-HOP study?