1. A noninvasive method for diagnosing the degree of vessel stenosis in a patient comprising the steps of: (a) comparing patient blood flow data, based upon time domain and frequency domain ultrasound Doppler shift information obtained at a Doppler angle .theta. defined with respect to the patient's common and internal carotid arteries, with corresponding reference blood flow data, characteristic of a test group of people whose common and internal carotid arteries have been independently evaluated for stenosis, the blood flow having characteristic regions designated systole, diastole, early diastole, late distole, and dicrotic notch, the patient blood flow data and corresponding reference blood flow data being compared defining a first set that includes information indicative of: (1) the logarithm of the product of the reciprocal of cos .theta. and the mean blood flow in the common carotid artery during early diastole; (2) the reciprocal of cos .theta. multiplied by the change in the blood flow frequency in the common carotid artery after systole, the product of which is raised to the fourth power; (3) the logarithm of the mode blood flow frequency in the internal carotid artery; (4) the minimum point on a signal contour, which is separated by approximately 9 dB from a contour representing the mode blood flow frequency in the internal carotid artery, raised to the fourth power; and (5) the logarithm of the product of the reciprocal of cos .theta. and the mean blood flow in the common carotid artery during later diastole; and (b) producing a diagnosis output, the diagnosis output indicating a diseased condition when the patient blood flow data and corresponding reference blood flow data have a first relationship and indicating a normal condition when the patient blood flow data and corresponding reference blood flow data have a second relationship.