Background: Major side effects after endovenous laser ablation (EVLA) are pain and bruising. Low temperature of the tumescence fluid might cause additional venous constriction and a cooling effect around the vein. The aim of this study was to show outcome and side effects after EVLA of incompetent great saphenous veins (GSV) with a 1470 nm Diode laser (Ceralas E, biolitec) using cold or warm tumescence fluid for anaesthesia. Patients and methods: Between August and November 2007, 85 consecutive patients (85 legs) with an incompetent GSV were treated by EVLA. The patients were randomized in two groups. In 42 patients (Group A) a warm (37 °C) and in 43 patients (Group B) a cold (5 °C) tumescence fluid (TF) was used for local anaesthesia in the track of GSV. All patients were re-examined after 1, 10 and 30 days clinically and by duplex for complications and occlusion in the treated vein segment. Patient’s satisfaction was assessed on a 0 to 4 points scale. Results: In each group one patient was lost to follow-up. There was no significant difference concerning gender, age, C of CEAP, BMI or diameter of the treated vein. In Group A a mean of 462 ml TF and in Group B a mean of 428 ml TF were used. In Group A the mean LEED (average linear endovenous energy density) was 114 J / cm and in Group B 115 J / cm. In both groups occlusion of the treated veins was achieved for all patients. The diameter of the GSV at 3 cm below the sapheno-femoral junction shrunk from 1.0 to 0.7 cm in both groups. The modified CEAP clinical score improved in Group A from 2.9 to 0.7 (mean value) and in Group B from 3.0 to 1.1. The mean pain score on a scale from 0 to 4 during day 2 to day 10 was 1.2 in Group A and 1.0 in Group B. At this time patients in Group A took a mean of 3.4 and in Group B 1.7 analgetic tablets. Ecchymoses were rare in both groups (4 in Group A, 7 in Group B). Conclusions: In this prospective randomized comparative study the temperature of the tumescence fluid did not influence the occlusion rate when a high LEED was used. In both groups pain and ecchymoses are less frequent in this study with a 1470 nm diode laser than reported in studies with 810-980 nm systems. Cold tumescence fluid reduced pain slightly and reduced the intake of analgetics significantly.