Aim of the study. To elucidate the mechanisms underlying the development of reflux and pathological venous capacity in the basin of great saphenous vein (GSV) based on the results of duplex ultrasound of the lower extremities in the patients with primary varicosis of the lower extremities. Material and methods. A total of 409 protocols of junction ultrasound (439 lower extremities) were available for the analysis. Results. The most characteristic findings included incompetence of the saphenofemoral (SFJ) and reflux in the GSV trunk. Vertical reflux accounted for 50.3% of the total cases. The total GSV incompetence was documented in 27% of the cases. Perforating veins were found to be incompetent in 49.7% of the patients; as a rule either one or two vessels were affected. Conclusion. The length of the incompetent GSV segment and the maximum diameter of the venous ectasia zones have substantial effect only on the registration of the Cokket's groups of incompetent perforant veins whereas the average GSV ostial diameter influences the development of reflux in all crural perforant veins with the exception of Sherman's perforator.