Endovenous laser treatment (ELT) is a minimally invasive technique used to treat varicose veins. In this technique, an optical fiber is inserted into the vein to be treated and the laser lights while the optical is slowly withdrawn. The average linear endovenous energy density (LEED) delivered during the procedure is determinant in the success of this therapy. Recanalization and side effects are energy-dependent. Thus, the success of the therapy is mainly related to the pullback speed of the fiber. Ideally this speed has to be 1mm/s. The purpose of this investigation was to evaluate in real time the fiber pullback speed. In order to measure in real time the motion of the fiber inside the saphenous vein, magnetic tracking was used. The system was evaluated during the treatment procedures for 18 patients. The mean pullback speed was 1.02±0.15mm/s similar to the speed the physician wanted to obtain. Based on quality control, safety standards, and regulations official documents for medical lasers, where a 20% accuracy is admitted; only one patient was treated with an average pullback speed inferior to 0.8mm/s (0.77cm/s) and two patients were treated with an average pullback speed superior to 1.2mm/s (1.26mm/s and 1.32mm/s). Consequently, even for a well-trained physician, variations can be observed. These variations could easily explain why complications (too low speed and consequently too high LEED) or recurrences (too fast speed and consequently too low LEED) are sometimes reported in clinical studies. A standardized or an automatic withdrawal of the fiber could possibly improve this technique but remains to be carefully evaluated.