Endovenous techniques such as radiofrequency ablation (RFA) and endovenous laser therapy (ELT) have emerged as percutaneous minimally invasive procedures for ablation of incompetent great saphenous veins in patients with varicosity and venous insufficiency. Early reports showed safety and efficacy of both techniques, with excellent technical success rates and few major complications, such as deep vein thrombosis or pulmonary embolism. During our initial experience with ELT in 56 limbs of 41 patients, 39 underwent postoperative duplex scanning. We encountered three cases (7.7%) with thrombus extension into the common femoral vein. All three patients were anticoagulated, and a temporary inferior vena cava filter was placed in one. All remained asymptomatic. The thrombus resolved by 1 month in all three patients. Review of the literature revealed that the incidence of thrombus extension into the common femoral vein or deep vein thrombosis in published clinical series is 0.3% after ELT and 2.1% after RFA. This possibility warrants routine postoperative duplex scanning, more alertness during these procedures, and patient education on this possible complication.