Adventitial cystic disease (ACD) is a rare, but well-characterized vascular disease. It is most commonly seen in the popliteal artery, but it has also been reported in the venous system. The most commonly involved segment has been the common femoral vein; the disease resulted in luminal compromise and extremity swelling. We report here on a case of adventitial cystic disease of the left external iliac vein that was initially misdiagnosed as deep vein thrombosis in a 68-year-old man who presented with a painless swelling of his left leg.