The association between male subfertility and varicocele has long been recognized. Physical examination is the most widely used method for detecting varicoceles but other techniques may be more sensitive. Color Doppler sonography has the theoretical advantage of allowing direct demonstration of reversed flow in the testicular veins. The frequency of detection of reversed flow by color Doppler ultrasound in a normal population is, however, unknown. We evaluated 34 asymptomatic young men with physical examination, standard scrotal sonography and color Doppler evaluation of the scrotal veins. Findings were correlated with semen analysis. Of the subjects 15% had varicoceles by physical examination and 18% by scrotal sonography, while 35% had retrograde flow by color Doppler evaluation. The majority of men in this population did not exhibit retrograde flow within the gonadal veins when evaluated by color Doppler sonography. The physiological significance of retrograde flow, however, remains to be defined.