Objectives: Central aortic systolic pressure (cSBP) has been shown to be more closely related to ventricular load compared to conventional brachial pressure. This study assesses a non-invasive method for the automatic measurement of cSBP based on the QKD interval, a measure of arterial stiffness, obtained from the time interval between the Q wave of the electrocardiogram and the diastolic heart sound of the pressure pulse detected in a brachial cuff. The QKD method is compared to the established SphygmoCor device for non-invasive estimation of cSBP. Methods: The QKD interval (Tq) was measured in 65 patients (age 34-85 years, 18 females) referred for cardiac catheterization (Diasys Integra II, Novacor, France) with simultaneous invasive measurement of cSBP and brachial cuff sphygmomanometric mean pressure (bMAP). A regression model was obtained for estimated cSBP (cSBPq) in terms of Tq (ms), bMAP (mmHg), heart rate (HR, b/min) and height (cm) as: cSBPq =120 +1.44xbMAP - 0.45xHR - 0.40xheight - 0.12xTq. This regression model was used on a test cohort of 107 subjects (age 15-87 years, 62 females) which had non-invasive cSBP estimated by the QKD technique (cSBPq) and simultaneously by the SphygmoCor (AtCor Medical, Australia) device (cSBPs). Results: For the whole cohort there was a strong correlation between cSBPq and cSBPs (r2 =0.94) with a mean difference (mmHg) of 2.5±5.5 (P<0.05). This difference varied with age groups: Young (mean age 28±5) 2.0±5.0 (P<0.05); Middle age (mean age 50±7): 4.5±4.5; Old (mean age 72±8): 0.5±7 (NS). All mean differences are in within 0.4% and 4%. Conclusion: This study showed that reliable measurements of cSBP can be obtained by simultaneous measurement of QKD interval and conventional cuff devices using regression equations with good comparison with other established techniques using a generalized transfer function. The QKD technique can be incorporated in ambulatory monitoring of cSBP.