Highlights. Ultrasound and MR venography of internal jugular veins in case of unilateral cerebellar hypoplasia or extrinsic stenosis enables noninvasively and without contrast agents differentiation of these conditions due to local and region-specific indicators, including the area and velocity of blood flow, arterial-venous balance, visualization of low blood flow, compression factor, and the overall clinical picture of venous network of the neck remodeling and the development of collateral circulation.Aim. To compare the visual and hemodynamic characteristics of ultrasound and magnetic resonance venography of extrinsic stenosis and hypoplasia of internal jugular veins (IJV).Methods. Ultrasound and magnetic resonance (MR) venography were performed in 47 patients with extrinsic stenosis and 23 patients with hypoplasia of one of internal jugular veins, and in the control group (30 patients).Results. In case of obvious vein compression and deformation and local stenosis (65% on average), there is a decrease in the vein cross-sectional area by 2 times compared with the contralateral vein, the blood flow velocity in the stenotic vein in acute cases increases (on average, up to 45 cm/s), and in chronic cases it significantly (p = 0.00001) decreases (to 13 cm/s on average). The criteria for hypoplasia are the small caliber of the vein (on average 0.22 cm², within the range of 0.13–0.46 cm²) throughout its entire length - with the cross-sectional area not exceeding that of the common carotid artery, and the vein being 4 times smaller than IJV on contralateral side, as well as a significant (p = 0.0006) velocity increase on the contralateral side (on average 40.9±12.52 cm/s) with normal velocity in the hypoplastic vein. Ultrasound assessment of the arterial-venous balance (AVB) makes it possible to evaluate the contribution of each IJV to the outflow of venous blood from the brain separately. From the hypoplastic side it was only 11.01±6.72%, and from the stenotic side it was 18.22±17 42% on average, which is significantly (p = 0.000001) below the norm, but does not allow us to differentiate these conditions. The total AVB for IJVs on both sides was within normal values (65–85%), and the absence of a pronounced expansion of the vertebral veins during MR venography confirms their insignificant role in the compensatory mechanisms of ensuring adequate outflow of venous blood from the brain in cases of unilateral hypoplasia and extrinsic stenosis of IJV.Conclusion. Ultrasound and MR venography of internal jugular veins in cases of unilateral hypoplasia or extrinsic stenosis enable noninvasively and without contrast agents differentiation of these conditions due to local and region-specific indicators, including the area and velocity of blood flow, arterial-venous balance, visualization of low blood flow, compression factor, and the overall clinical picture of venous network of the neck remodeling and the development of collateral circulation.