Symptom guide
Knee swelling: causes, red flags and when to get it checked
A swollen knee means extra fluid in the joint (an effusion) or swelling of the tissues around it. Injury, osteoarthritis and inflammatory conditions such as gout are common causes. A hot, red, very painful knee with fever should be seen the same day, since infection must be excluded and treated quickly.
Last checked: 2026-10-03
Key facts
At a glance.
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Timing
Swelling within an hour of injury suggests bleeding into the joint; swelling after a day suggests fluid from irritation.
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Hot joint
A hot, red, very painful knee with fever may be septic arthritis, an emergency.
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No injury
Gout, pseudogout, osteoarthritis flares and inflammatory arthritis are common causes of swelling without injury.
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Baker cyst
A swelling behind the knee may be a Baker cyst, but calf swelling needs a clot excluded.
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Who to see
GP or urgent care; orthopaedics or rheumatology as needed.
What this symptom usually means
The knee joint is lined by a membrane that produces a small amount of lubricating fluid. When the joint is injured or inflamed, this membrane makes more fluid, or blood enters the joint, causing swelling. Swelling can also be outside the joint, in a bursa or in the soft tissues.
Possible causes, from most to least common
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Injury: ligament sprain, meniscus tear or ACL tear, with swelling soon after a twist or impact.
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Osteoarthritis flare: gradual swelling with pain, stiffness and grinding, worse after activity.
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Overuse and bursitis: prepatellar (housemaid) bursitis from kneeling, with swelling over the kneecap.
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Gout and pseudogout: sudden hot, red, swollen, very painful knee.
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Inflammatory arthritis such as rheumatoid and psoriatic arthritis, reactive arthritis after infection.
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Baker cyst: fluid behind the knee, often with underlying arthritis or meniscus tear.
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Septic arthritis: hot, swollen, very painful knee with fever and inability to move it.
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Deep vein thrombosis, cellulitis, bleeding disorders on blood thinners and, rarely, tumours.
Features that point to something more serious
A hot, red, swollen, very painful knee, fever or rigors, inability to bear weight or straighten the knee, swelling after significant injury, locking, a swollen calf, a history of joint replacement, immunosuppression, or bleeding disorder need urgent assessment.
Red flags: when to seek help
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A&E or same-day GP: a hot, red, severely painful knee with fever, especially after injury, with a joint replacement, or on immunosuppressants.
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A&E or urgent care: swelling after a significant injury with inability to bear weight, or deformity.
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Same-day assessment: swelling of the knee with a swollen painful calf.
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GP within days: a swollen knee for more than a few days with no clear cause.
What a doctor will ask and examine
The doctor asks when swelling began, injury, pain, locking, giving way, fever, previous gout, arthritis, skin conditions, bowel symptoms, sexual exposure and medicines. They look for redness, warmth, effusion (patellar tap), range of movement, ligament stability and examine the calf.
Tests that are commonly used and why
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X-ray: for osteoarthritis, fracture and calcification.
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MRI: for ligament and meniscal injury or persistent unexplained swelling.
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Joint aspiration with crystal analysis and culture: to diagnose gout, pseudogout and infection.
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Blood tests: inflammatory markers, uric acid and rheumatoid markers.
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Ultrasound: to assess fluid, cysts and rule out DVT in the calf.
Self-care and the usual route
Remember PRICE principles for injury: protect, rest briefly, ice for 15 to 20 minutes, compression with a bandage and elevate. Gentle movement as pain allows prevents stiffness, and simple painkillers or topical anti-inflammatories can help. Weight management and strengthening reduce osteoarthritis flares. Do not ignore a hot, painful swollen knee.
The NHS route is a GP, urgent care or A&E, with physiotherapy, orthopaedics or rheumatology as needed. Private orthopaedic and rheumatology clinics offer direct access to MRI, aspiration and injections.
Frequently asked
Questions people ask.
Quick, plain-English answers to the questions we hear most.
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What causes a swollen knee?
Injury, osteoarthritis flares, gout, inflammatory arthritis, bursitis and infection are common causes.
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When is a swollen knee an emergency?
When it is hot, red and very painful with fever, or after a significant injury with inability to bear weight.
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What is water on the knee?
A common term for knee effusion, meaning excess fluid in the joint, which can result from many conditions.
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Can gout affect the knee?
Yes. Gout can cause sudden, hot, red, swollen knees, confirmed by joint fluid analysis.
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Should I have the fluid drained?
Sometimes, to relieve pressure and to test the fluid, particularly if infection or crystals are suspected.
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Will an MRI be needed?
For injuries with locking or instability, or persistent unexplained swelling, an MRI is often useful.
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What helps swelling at home?
Rest, ice, compression and elevation initially, then gradual movement, and simple painkillers.
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What is a Baker cyst?
A fluid-filled swelling behind the knee connected to the joint, often associated with arthritis or a meniscal tear.
Where this comes from
Sources and checking.
This guide is for information, not medical advice.
Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.
Published 2026-10-03, next review 2027-10-03.
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NHS: Knee pain (nhs.uk/conditions/knee-pain/)
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NHS: Gout (nhs.uk/conditions/gout/)
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NICE guideline NG226: Osteoarthritis in over 16s - diagnosis and management