Objective: to study the extent of venous reflux in the great saphenous vein (GSV) in patients with chronic venous insufficiency (CVI) of the lower extremities at different stages of the disease and the possibility of its influence on surgical tactics. Materials and methods. The study included of 43 patients with CVI of the lower extremities (47 limbs) treated in the surgical department of the Yu. Lypa Lviv Regional Veterans Hospital. The average age of patients was 47 years. There were 40 (85.1%) class C2–C3 patients and 7 (14.9%) class C4–C6 according to the clinical classification of CEAP. The patients underwent endovenous laser ablation of the GSV in the segment with venous reflux and miniphlebectomy of varicose branches under totally local anesthesia. Results. In the vast majority of patients, venous reflux was determined only to the border of the upper and middle third of the tibia — 27 (57.4%) limbs, and along the entire length (axial) in only 3 (6.4%) limbs. In 5 (10.6%) limbs, venous reflux was determined only to the middle third of the thigh, in 4 limbs (8.5%) to the lower third of the thigh, in 3 limbs (6.4%) to the knee level, and in 5 (10.6%) to the lower third of the shin. In accordance with the ultrasound picture and the extent of the venous reflux in the GSV, endovenous laser ablation was performed in their insufficient segments only, preserving the distal unaffected segments. Conclusions. Targeted ultrasound study of the extent of the venous reflux in the GSV in patients with CVI of the lower extremities allows endovenous laser ablation to be performed in the insufficient segments of the vein only, while preserving the unaffected ones and reducing the traumatic nature of the interventions without reducing their effectiveness.