The aim of the study was to establish ultrasound criteria of the differentiation between functional and morphological vasoconstriction of the vertebral artery (VA) using hypercapnia test in patients with signs of vertebral-basilar insufficiency. The inclusion criteria were the presence of clinical symptoms of vertebral-basilar disturbances of circulation and small diameter of VA (≤2.9 mm). Fifty patients, aged from 17 to 57 years, were studied. Based on the form of Doppler 's spectrum of the blood flow in the small VA, patients were stratified into three groups: normal (N spectrum), peak-like split (PS spectrum) and high-resistance low-speed (HL spectrum). The increment of diameter by 0.2 mm and more, i.e. the functional vasoconstriction (spasm) during hypercapnia, was characteristic of patients with PS and N spectrum. The ROC-analysis revealed that the absence of VA increment during hypercapnia is a highly specific (90.9%) predictor of morphological vasoconstriction of VA. Quantitative criteria of VA hypoplasia were specified as well. Ultrasound angioscanning in the combination with hypercapnia test allows differentiation with a high probability between functional (spasm) and morphological (hypoplasia) vasoconstriction of VA.