OBJECTIVE To assess changes in the great saphenous vein (GSV) after foam sclerotherapy for varicose veins. METHODS This was a prospective study of 33 patients who were treated with polidocanol foam sclerotherapy after having had varicose veins with a clinical severity-etiology-anatomy-pathophysiology classification of C4-C6 for three months. The patients were evaluated by ultrasound before, during, and after the procedure (on post-procedure days 7, 15, 30, 60, and 90). The GSV diameter, the rate of venous occlusion, and the rate of reflux elimination were determined. Two patients were excluded for having a history of deep vein thrombosis history, and one was excluded for having bronchial asthma. RESULTS Thirty patients (26 females and 4 males, with mean age of 62 years) completed the protocol. The mean pre-procedure GSV diameter was 6.0 ± 0.32 mm (range, 3.6-11.2 mm). During the sclerotherapy, the mean GSV diameter was reduced to 1.9 ± 0.15 mm (range, 0.6-3.8 mm). On post-procedure day 7, the mean GSV diameter increased to 6.3 ± 0.28 mm (range, 3.9-9.7 mm). On post-procedure day 90, the mean GSV diameter was 4.0 ± 0.22 mm (range, 1.9-8.2 mm). The rate of GSV reflux was significantly lower in the assessment performed on post-procedure day 90 than in the pre-procedure assessment (p < 0.0028). CONCLUSIONS On the basis of our ultrasound analysis, we can conclude that foam sclerotherapy for varicose veins results in a significant reduction in GSV diameter, as well as in the elimination of GSV reflux.