The aim — to study morphological and ultra‑sonic changes in the thrombosed magistral vein after the high‑frequency endovascular electric welding and to determine the effectiveness of the method for the acute ascending thrombophlebitis treatment.Materials and methods. Surgical treatment of 52 male and female patients with acute ascending thrombophlebitis of the great subcutaneous vein was performed in the surgical departments during the period from 2016 to 2018, in which the endovenous high‑frequency electro welding with the EK300M Svarmed (Ukraine) apparatus was done. Their age ranged from 19 to 78 years old (average age — 51.28 ± 2.63 years). С2 stage was registered for 4 patients, С3 — for 19, С4 — for 13, С5 — for 9, С6 — for 7 according to the international CEAR (Clinical Etiological Anatomical Pathophysiological) classification. The morphological study of sections was carried out. Photos were archived with the light optical microscope Zeiss (Germany) and the Axio Imager. A2 data processing system. Ultrasonic duplex angioscanning was carried out by the Toshiba Nemio XG (Japan) device with a curvilinear transducer with at working frequency of 3.5 — 5 MHz and a linear transducer in the range of 7.5 — 12 MHz. The ultrasound examination was performed in 2 — 7 days after the surgery, the remote results were evaluated in 3.6.12 months.Results and discussion. A morphological study of vein preparations revealed homogenization of thrombotic masses in the vein’s lumen, a close connection of the latter with the treated vessel wall. In most cases, the thrombotic masses totally obstructed the lumen of the vein. All layers of the vein wall merged into a single homogeneous complex. With specific coloring on elastic fibers, their total destruction in the inner and middle layers, thinning and fragmentation of adventitia elastic fibers were revealed. Ultrasound examination showed complete occlusion of the great saphenous vein on the 2nd — 7th day in 49 (94.24 %) cases. Partial recanalization was detected after 3 — 6 months in 3 (5.76 %) cases, after 9 — 12 months in 2 (3.84 %) cases the partial hemodynamically insignificant recanalization was preserved according to color mapping. In 1 case (1.92 %) there was a complete, hemodynamically significant recanalization of the whole welded segment of the great subcutaneous vein. The analysis of ultrasonic testing showed that, despite the complete destruction of the entire vein wall, during the first days after the endovascular electric welding in 5.76 % of the welded great subcutaneous vein, areas of the wall blood flow were detected.Conclusions. Endovascular high‑frequency electrical welding of the thrombosed great subcutaneous vein for acute ascending thrombophlebitis causes complete destruction of the thrombotic masses and all layers of the vein wall; and in 94.24 % of cases it allows achieving a complete obliteration of the welded vein.