The accepted view that varicose veins are primarily due to valvular incompetence is challenged and the alternative theory of weakness of the vein wall is supported by the correlation of clinical, operative and histological findings. It is shown that there appears to be a basic abnormality of the muscle cell associated with abnormal collagen infiltration. The morphological changes in the vein wall are demonstrated by light microscopy and transmission electron microscopy. It is thought that the cause of these changes are to be sought in an investigation of the enzyme systems involved in maintaining tissue equilibrium in the vein wall.