A method and apparatus for non-invasively determining a physiological condition. The method can include steps of monitoring peripheral arterial tone using an external sensor, detecting a change in the peripheral arterial tone, and determining the physiological condition when a specific change in the peripheral arterial tone has been detected. The physiological condition may be a cardiovascular disease state or endothelial dysfunction, for example. The apparatus can include a probe applied to a subject's digit for sensing peripheral arterial tone and outputting a corresponding signal, and a processor which processes the signal and provides an output from which the physiological condition can be determined or which determines the physiological condition based on the signal.