Aim — the objective of the present study was the evaluation of the effectiveness and safety of endovenous laser ablation (EVLA) for the correction of the compromised straight segment of a large side branch of the great saphenous vein in the thigh. Material and methods. This prospective non-comparative study included 315 patients who had undergone a total of 343 EVLA interventions on the great saphenous vein and their side branches. The parameters of the laser used for the purpose were as follows: wavelength 1470 nm, power from 6 to 10 W, linear energy density from 66.7 to 100 J/cm. The patients were examined 2, 6 and 12 months after the treatment. The technical result (vascular occlusion) was estimated within 2 and 6 months after the intervention with special reference to the presence or absence of complications, such as the retraction of the skin or the subcutaneous adipose tissue, hyperpigmentation, the development of phlebitis with the pronounced pain syndrome within 12 months after the intervention. Results. The diameter of the side branches of the great saphenous vein varied from 5 to 14 (mean 9±2.3) mm. The duration of observation ranged from 72 to 357 days. The positive technical results apparent within 2 months after the treatment were documented in a total of 340 (99.1%) cases. Recanalization of the side branches and hyperpigmentation along the length of the vessel occurred in 3 (0.9%) and 6 (1.7%) observations, respectively. Five (1.5%) patients developed the local retractions of the skin and subcutaneous adipose tissue in the zone of obliteration; in two (0.6%) cases, these features persisted throughout the long-term follow-up period. Conclusion. Endovenous laser ablation can be recommended for the treatment of the straight portion of the major side branch of the great saphenous vein in the thigh.