The Windkessel function of elastic arteries determines the postpeak contour of the internal carotid artery (ICA) Doppler waveform. To introduce an indicator for the function, the postpeak contour was reproduced by an exponential function, and the exponential coefficient was termed "Decay Index" (DI). DI obtained from 108 stenosis-free ICAs (55.2 ± 15.8 y) was compared with surrogate measures for the Windkessel function (aortic pulse wave velocity, pulse pressure) and ICA peripheral resistance (resistive index, pulsatility index, systolic-to-diastolic ratio). DI was moderately correlated with the Windkessel and the resistance measures (r ranged 0.44–0.57). To clarify these intercorrelations, principal component analysis revealed two components. The first was loaded by the resistance measures (component load = 0.93) although the second was loaded by the Windkessel measures and DI (= 0.70), supporting an association between DI and the Windkessel function. Thus, DI is likely to represent an indicator for the Windkessel function.