Duplex ultrasound scanning of the penis combined with intracorporeal pharmacological stimulation of an erection provides an accurate, relatively noninvasive functional assessment of cavernous artery blood flow. Various criteria described for normal have been based on either blood flow velocity alone or the combination of blood flow velocity and arterial dilatation of each cavernous artery. Unfortunately, less than 20% of impotent men have classical arterial anatomy and the aforementioned normal criteria do not take into account the various possible anatomical and acquired anomalies that can exist. In addition, the multiplicity of parameters used to describe normal are cumbersome to use for patient comparison and statistical analysis. In this study, 4 indexes were developed to express the results of a duplex ultrasound penile blood flow study and they were tested in an impotent population comprised of 75 men with arterial disease and 33 men with impotence due to neurogenic or psychogenic causes. A significant difference was observed between these 2 groups using all 4 indexes (p less than 0.001). However, the penile blood flow index, which incorporated velocity and dilatation, performed best as determined by sensitivity, specificity and receiver operating characteristics. Based on the results of this study, the penile blood flow index, which represents the summation of the percentage of dilatation and peak blood flow velocity of each cavernous artery, provides an accurate reflection of total penile blood flow as measured by duplex ultrasound.