Having varicose veins on just one leg is actually the more common pattern. Roughly 80% of people with varicose veins have them on one side only. While varicose veins always involve faulty valves that let blood pool and stretch the vein walls, the reason those valves fail in one leg and not the other usually comes down to something specific on that side: a past blood clot, compression of a vein in the pelvis, or simply a structural quirk you were born with.
How Varicose Veins Develop on One Side
Your leg veins rely on tiny one-way valves to push blood upward against gravity. When those valves weaken or get damaged, blood flows backward and pools in the vein, causing the bulging, ropy appearance of varicose veins. In bilateral cases, the underlying cause tends to be something systemic like genetics, obesity, or prolonged standing that weakens valves in both legs at a similar rate. When only one leg is affected, something local is usually tipping the balance on that side.
The most common local triggers fall into a few categories: a blood clot that scarred the valves in one leg, a vein being physically compressed in the pelvis, pelvic vein problems that drain into one leg more than the other, or a congenital vascular condition. Each of these creates a bottleneck or damage point that’s specific to one side of the body.
A Past Blood Clot Is the Leading Cause
Deep vein thrombosis, a blood clot in one of the deep veins of your leg, is the single most important risk factor for chronic venous insufficiency. Even after the clot dissolves or is treated, it leaves behind scar tissue that damages the valve inside that vein. The scarred valve can no longer close properly, so blood leaks backward and pressure builds in the veins below it. This is called post-thrombotic syndrome.
Because DVT almost always strikes one leg, the valve damage it causes is one-sided. You may not even remember having a clot. Some DVTs cause obvious swelling and pain, but others are “silent,” producing only mild symptoms that resolve on their own while still leaving lasting valve damage. If your varicose veins appeared months or years after a period of leg swelling, prolonged bed rest, surgery, or a long flight, a prior clot may be the explanation.
Pelvic Vein Compression (May-Thurner Syndrome)
In your pelvis, the main artery carrying blood to your right leg crosses over the main vein draining blood from your left leg. In most people this crossover is harmless. But in May-Thurner syndrome, the artery presses hard enough on that left-sided vein to partially flatten it, like stepping on a garden hose. Blood has a harder time flowing freely out of the left leg, which raises pressure in the veins below and can lead to varicose veins, swelling, or even a blood clot, all on the left side only.
May-Thurner syndrome is underdiagnosed because the compression can be mild enough to cause subtle symptoms for years before varicose veins become visible. It’s worth considering if your varicose veins are specifically on your left leg, especially if you also notice left-leg heaviness or swelling that worsens with standing. Imaging of the pelvic veins can confirm the diagnosis.
Pelvic Congestion and Ovarian Vein Issues
Varicose veins that appear on one leg after pregnancy, or that seem to start high on the inner thigh or around the vulva, can originate from veins in the pelvis rather than from a problem in the leg itself. Enlarged pelvic veins, sometimes called pelvic congestion syndrome, allow blood to flow backward through a network of connections into the leg’s superficial veins. These connections can drain into the main superficial vein of either leg, but the drainage pattern is often asymmetric, producing varicose veins on one side.
Varicose veins extending over the buttock and inner thigh that connect with the superficial veins of the leg are found in roughly 4 to 9% of people with lower-extremity venous insufficiency. When standard vein treatments on the leg don’t fully resolve the problem, the source may be a leaky pelvic vein feeding into that leg from above.
Congenital Vascular Conditions
Rarely, one-sided varicose veins are part of a condition you were born with. Klippel-Trenaunay syndrome is the most recognized example. It involves abnormal development of blood vessels and soft tissue in one limb, and doctors diagnose it when at least two of three features are present: flat red or purple birthmarks (port-wine stains) on the skin, varicose veins or other vein abnormalities, and overgrowth of bone or soft tissue making one limb noticeably larger than the other.
Port-wine stains appear in 90 to 100% of cases, and vein abnormalities show up in 70 to 100%. If you’ve had a visibly larger leg or skin discoloration on that side since childhood, this is a possibility your doctor can evaluate. It’s uncommon in the general population, but it’s one of the clearest explanations for varicose veins that have always been limited to one leg.
How Doctors Pinpoint the Cause
The standard first step is a duplex ultrasound, a painless scan that combines a live image of your veins with measurements of blood flow direction and speed. The technician typically examines the full length of your leg from the groin to the ankle, checking each major vein for backward flow (reflux) and for signs of old clot damage. When reflux is suspected, they’ll use calf squeezes or other maneuvers to provoke blood flow and then measure how long it takes for blood to reverse direction, reported in seconds or fractions of a second.
For one-sided varicose veins, the scan pays particular attention to the junction where the main superficial vein meets the deep vein at the groin, because this is the most common site where valves fail. If the ultrasound doesn’t explain the pattern, or if a pelvic source is suspected, further imaging of the iliac veins in the pelvis can check for compression or obstruction higher up.
Treatment Outlook for One-Sided Varicose Veins
The good news about having varicose veins on only one leg is that you’re dealing with a more contained problem, and treatment is focused on that single side. The most common modern approach uses heat energy delivered through a thin catheter inside the vein to seal it shut, after which blood reroutes through healthy veins. Anatomical success rates for these procedures exceed 88% at one year. Over five years, some degree of recurrence can develop, with rates ranging from about 19% to 39% depending on the technique, but recurrence often means new small veins rather than a return to the original severity.
If the underlying cause is something structural like pelvic vein compression, treating only the visible leg veins may not be enough. Addressing the upstream problem, for instance with a stent to open a compressed iliac vein, or with a procedure to close a leaking pelvic vein, often improves the long-term outcome and reduces the chance of veins coming back on that side.
Knowing why the problem is one-sided matters for choosing the right treatment. A vein specialist can use your ultrasound findings, medical history, and the specific location of your varicose veins to determine whether the cause is local valve damage, a pelvic issue, or something else, and tailor the approach accordingly.