An arterial stiffness index based on the transmission line model is defined. The parameters of the transmission line model are initially estimated using measured pressure, flow and aortic root diameter. Pressure is measured at the carotid using applanation tonometry. Flow is measured using Doppler at the ascending aorta. Aortic root diameter is measured using 2-D echocardiography. The initial estimates are then refined using grey-box identification. The resulting estimate of the distributive compliance of the transmission line model is proposed to be an arterial stiffness index. Similar to the Windkessel compliance, this index describes the global stiffness. However, it is based on a more realistic 1-D model that can simulate wave propagation and wave reflection.