The term aneurysm is used to indicate a permanent and irreversible localized vascular dilatation that involves all three layers of the blood vessel wall. It may develop in any part of the circulatory system, including veins, and its history, presentation, and management differ depending on the location. A venous aneurysm is defined as a solitary area of fusiform or saccular dilatation occurring in the course of a major vein or connected by a single channel to a major venous structure. The lower extremities are the most frequently affected, with the popliteal vein being the most common site. Although different theories have been advanced, the etiology of the disease remains uncertain. Mechanical stress and/or degenerative changes within the vein wall are believed to represent the most likely causes of venous aneurysm. To date, there are only a few publications dealing with the histological appearance of popliteal vein aneurysm, and no studies that specifically describe its ultrastructural details have been published to our knowledge. In an attempt to fill this gap and to provide better insights into the pathophysiological mechanisms possibly involved in aneurysmal venous disease, we describe the fine structure of popliteal vein wall and valve in health and disease using both scanning and transmission electron microscopy.