Distinguishing stenosis and hypoplasia of the extracranial vertebral artery (VA) is clinically important, but there is no agreement in defining VA hypoplasia. We endeavored to attain reference values for VA flow volume by color Doppler ultrasonography (CDU), analyze age and gender effects on VA flow volume and develop a definition of VA hypoplasia. CDU was performed in 447 subjects free of cerebrovascular diseases or carotid stenosis. The VA diameter, peak systolic velocity, end-diastolic velocity, time-averaged mean velocity, resistance index (RI) and flow volume were recorded bilaterally. We found significant asymmetries in diameter, flow velocities and flow volume with left-sided dominance. Diameters were different on left (0.297 +/- 0.052 cm) and right (0.323 +/- 0.057) sides (p < 0.001). Flow volume was different on right (83.0 +/- 36.9 mL/min) and left (96.6 +/- 42.4) sides (p < 0.001). Women had significantly smaller diameters, higher flow velocities and lower RIs than men. VA flow volume did not change with aging. We defined hypoplasia as a significant decrease in flow velocities and increase in RI for VA diameters < 0.22 cm. This definition is supported by findings of an increase in ipsilateral flow resistance (RI >/= 0.75), contralateral diameter (side-to-side diameter difference >/= 0.12 cm), and flow volume (side-to-side flow volume ratio >/= 5). This definition and these reference values may lead to better differentiation between congenital variation and steno-occlusion clinically.