Lump or Swelling Behind Your Knee? What is a Popliteal Vein Aneurysm?

If you have felt a lump behind your knee or noticed that one leg keeps swelling for no clear reason, your first thought probably was not your veins. Most people associate vein trouble with varicose veins or spider veins, not with a deep vein behind the knee. But a rare condition called a popliteal vein aneurysm can show up exactly this way, and it is worth understanding because it can carry serious risks if it goes unchecked. Here is what it is, why it matters, and when to get it looked at.

Take the Next Step at Center for Vein Restoration

Not sure if your symptoms are worth checking?

A little swelling or a lump behind the knee is often nothing serious, but it is always better to know. Our board-certified vein specialists at Center for Vein Restoration, the nation's largest physician-led vein care practice with more than 130 locations, can give you a clear answer. 

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Had an abnormal ultrasound result or unexplained one-sided swelling?

Do not sit with the uncertainty. If a scan flagged something, or one leg keeps swelling without explanation, our vein specialists can review your imaging, walk you through what it means for you, and answer your questions. We’re here to help.

Concerned about a clot or your current risk?

If you have a history of unexplained clots or pulmonary embolism, timely evaluation is important. Find a Center for Vein Restoration vein specialist near you so we can help protect your long-term vein health. If you are having sudden chest pain or trouble breathing, call 911 instead.

Frequently Asked Questions

Q: Is a lump behind my knee always something to worry about?
A: No, a lump behind the knee has many possible causes, and most are not dangerous. Still, a new or growing lump is worth having checked by a specialist so any vein-related cause can be ruled in or out.

Q: Can a popliteal vein aneurysm go away on its own?
A: These aneurysms do not typically resolve on their own, but not all of them require treatment. Depending on the size, shape, and your risk factors, a specialist may recommend simply monitoring it over time with regular ultrasounds.

Q: How would I even know if I had a popliteal vein aneurysm?
A: Many people have no symptoms and only find out during an ultrasound done for another reason. When symptoms do appear, they often show up as one-sided leg swelling, aching behind the knee, or a noticeable fullness in that area.

Q: What is the risk if a popliteal vein aneurysm goes untreated?
A: The main concern is that a clot can form inside the aneurysm and travel to the lungs, causing a pulmonary embolism, which can be life-threatening. This risk is exactly why evaluation by a vein specialist is worthwhile, even when you feel fine.

Mohamed T. Hassan, MD, DABVLM, RPVI, is a fellowship-trained physician specializing in venous and lymphatic disease at Center for Vein Restoration. He completed his fellowship in Venous and Lymphatic Medicine at Center for Vein Restoration and is board-certified in venous and lymphatic medicine. Dr. Hassan is dedicated to diagnosing and treating venous disease, with a focus on patient-centered, evidence-based care.