Background: It is important to acquire technical knowledge about the power and linear endovenous energy density (LEED) settings needed to achieve the ultimate goal of endovenous laser ablation (EVLA). Objectives: To evaluate the influence of different LEEDs in terms of patency and presence of reflux and to determine clinical outcomes. Methods: Sixty great saphenous veins (GSVs) were included. Patients were randomized into 2 groups, low-power EVLA (7 W and LEED of 20-40 J/cm) and high-power EVLA (15 W and LEED of 80-100 J/cm). Patients were followedup with duplex ultrasound and calculation of venous clinical severity score (VCSS) at 3-5 days, 30 days, 180 days, and 1 year after the procedure. Results: 18 patients (29 limbs) treated with 7 W of laser power and 13 patients (23 limbs) treated with 15 W of laser power completed the study. There was no significant difference regarding age, operating time, use of analgesics, laterality, sex, or presence of comorbidities. Mean LEED was 33.54 J/cm in the 7-W group and 88.66 J/cm in the 15-W group. Both groups exhibited improvements in VCSS and significant reductions in SFJ diameters, and there were no significant difference in increase of length of the GSV stump or rates of reflux after treatment. Conclusions: The higher energy density setting was more effective for stabilizing the length of the GSV stump and was associated with a lower incidence of reflux at 6 months. Further studies with a longer follow-up period are required to substantiate this hypothesis.