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Background: Augmentation index (AIx) is regarded as a marker of systemic arterial stiffness. Patients with type 2 diabetes mellitus (T2DM) have increased arterial stiffness, but not AIx, which suggests that mechanisms contributing to AIx in T2DM may differ from healthy individuals and be unrelated to arterial stiffness. The aim of this study was to examine the cardiovascular and clinical determinates of AIx (including arterial stiffness) in patients with T2DM compared with controls. Methods: Clinical characteristics and haemodynamic variables (including aortic and brachial pulse wave velocity [stiffness], cardiac output, systemic vascular resistance and heart rate) and AIx (by radial tonometry) were recorded in 53 T2DM and 53 matched controls. Correlates of AIx unadjusted for heart rate were assessed by uni- and multi-variable analysis. Results: Compared with controls, T2DM patients had significantly higher aortic stiffness (7.6 ± 1.6 vs 6.7 ± 1.9 m/s p = 0.016), cardiac output, heart rate, brachial and central BP; lower brachial stiffness and systemic vascular resistance, but no significant difference in AIx (27 ± 9 vs 24 ± 11% p = 0.184). AIx (adjusted or unadjusted) was not significantly related to aortic or brachial stiffness in either group (p > 0.198 all). Independent predictors of AIx in T2DM patients were height and heart rate, whereas in controls, AIx was independently related to height. Conclusions: Determinants of AIx in patients with T2DM differ from healthy individuals. Moreover, AIx is not significantly related to regional large artery stiffness and should not be regarded as indicative of systemic arterial stiffness.