Objectives: to evaluate the association of the greater saphenous vein (GSV) diameter in the treatment of patients with severe chronic venous insufficiency (C6 CEAP classification) with ultrasound-guided polidocanol foam sclerotherapy (UGFS). Methods: A prospective, descriptive and analytical study of 28 patients (30 limbs) that underwent UGFS. Patients were divided into 2 subgroups by GSV diameter (< 8 mm and ≥ 8 mm). Variables analyzed were ulcer healing, clinical intercurrences, clinical CEAP classification, Venous Clinical Severity Score (VCSS), diameter of the treated vein and presence of occlusion or recanalization by Doppler ultrasound. Patients were analyzed at the 1st, 3rd, and 6th months post-treatment. Results: The average age was 68.7 ± 10.5 years, 23 (82,1%) were women, and the average body mass index was 29.2 kg/m2. Although an improvement in VCSS score was observed during follow-up, no significant intergroup difference was noted. Seventeen (56%) limbs presented occlusion of the treated vein at the 1st month, 11 (36%) at the 3rd month, and 9 (30%) at the 6th month of follow-up. The ulcer healing rate was 56,6%. The average ulcer healing time was 90 days. Three (10%) patients presented with ulcer recurrence at the 6th month. Survival analysis showed no significant difference in ulcer healing rate between subgroups after one year of follow-up (log-rank, p = 0,178). Conclusion: There was no difference between the subgroups of large and small VSM diameter in terms of symptom severity. However, significant reduction of VCSS and pain relief was observed after foam sclerotherapy.