The larger more superficial veins are the ones most often affected by venous disease. Because the deeper veins are supported firmly by muscle tissue they do not tend to develop varicosities.

Leaky venous valves can occur at any site in the leg, but the great majority of varicose veins are caused by faulty valves in the groin or behind the knee. At both these sites there is a major junction at which the superficial veins flow blood into the important deep veins of the leg. The flow of blood at this junction is controlled by a one-way valve which is designed to direct blood inwards toward to larger deep veins and upwards, helping it to return to the heart.

Because the long saphenous vein is one of the main superficial veins in the thigh, it has many smaller, lesser veins flowing into it. When extra blood is forced into it by a leaking valve at the sapheno-femoral junction, the vein stretches and further valves become distorted by the pressure and begin to leak as well. Blood is then allowed to flow in the wrong direction further down the leg, eventually filling and distending more and more branches, causing the appearance of varicose veins in the thigh and calf.

A few patients will develop varicose veins as a result of malfunctioning valves in sites other than the groin or behind the knee, but regardless of where the leaking valves originate the more superficial veins below the site will eventually begin to protrude.

Spider veins, unlike varicose veins, most often do not occur in response to increased pressure except for the pressure created by nearby varicosities themselves. Spider veins occur most commonly in the thighs, ankles and feet but are often seen in the face and other areas of the body.