Comparative data on the effectiveness of various regimes of endovasal laser obliteration (EVO) on the veins of different diameter are scarce which complicates standardization of this method and accounts for their poor reproducibility. Aim of the study. To identify optimal energy regimens and EVO validity limits for the evaluation of the effectiveness of the method with the use of the composite end point. Material and methods. A multicenter retrospective longitudinal cohort study was carried out that included 257 patients with primary varicosis in the great saphenous vein (GSV) basin (CEAP C2-C3 classes). The patients were divided into two groups. Group "H" was comprised of 135 patients who underwent EVO with the use of a 970 nm wavelength laser. Group "W" contained 122 patients treated with the use of 1470 nm and 1560 nm lasers. The results of the treatment were estimated using the composite end point that included the following three components: pain, subcutaneous hemorrhages, and the frequency of GSV recanalization in the mid-third of the femur. Results. The study demonstrated the linear correlation between the outcome of the treatment and the wavelength of laser radiation in the midwave range (r=0.438 at p<0.001). The non-linear relationship was documented between the treatment outcome on the one hand and the linear energy density (LED) (p<0.001), venous diameter (p<0.001) and the patient's age (p=0.0278) on the other hand. The median of the applied LED was 87.5 J/cm (interquartile range 60.3-100 J/cm) and 92.5 (interquartile range 70-120 J/cm) in groups "W" and "H" respectively. This difference was significant (p=0.008). The optimal LED values and vein diameters were determined at which the best results of the treatment can be achieved. Conclusion. The optimal EVO regimes have been obtained under the following conditions: wavelength of 970 nm and venous diameter up to 8 mm with LED equaling 60-80 J/cm; wavelengths of 1560 nm and 1470 nm and venous diameter up to 10 mm with LED equaling 60-90 J/cm. The reproducibility of the method decreases in the case of laser obliteration of the veins with the diameter over 10 nm regardless of the wavelength and LED values. Simultaneously, the likelihood of the unfavourable outcome increases. Most outcomes were unfavourable in the case of laser obliteration of the veins with the diameter over 10 nm regardless of the wavelength and LED values. The results obtained in the present study need to be confirmed in a prospective comparative randomized trial.