Fluid on the knee: causes, symptoms and what to do
Fluid on the knee, or joint effusion, is an increase in synovial fluid inside the joint. It is not a disease in itself: it is a sign that something is irritating or injuring the knee.
Educational information prepared by Dr. Belinda Martínez, physician and surgeon. Last updated: October 3, 2026. It does not replace an assessment by a doctor.
What is fluid on the knee?
The knee normally produces a small amount of synovial fluid that lubricates the cartilage. When the joint is irritated, inflamed or injured, it produces more fluid and the knee swells. Sometimes the fluid contains blood (hemarthrosis), for example after a ligament injury or a fracture.
Most common causes
- Osteoarthritis: wear irritates the synovium and produces fluid, especially after activity.
- Meniscus or ligament injuries.
- Blows or falls.
- Arthritis: gout, rheumatoid arthritis or other inflammatory diseases.
- Joint infection (septic arthritis), which is an emergency.
- Bursitis, which looks similar but is fluid in a sac outside the joint.
Symptoms
- Visible swelling, sometimes only on one side of the kneecap or above it.
- Stiffness and difficulty fully bending or straightening the knee.
- A feeling of tightness or heaviness.
- Warmth and redness if there is inflammation or infection.
How it is diagnosed
The exam distinguishes whether the fluid is inside the joint. Ultrasound helps confirm it and an X-ray or MRI looks for the cause. When infection, gout or arthritis is suspected, the doctor may draw a sample of the fluid (arthrocentesis) for analysis.
What is done depending on the cause
Management aims to treat the cause and control inflammation: relative rest, ice, medications prescribed by the doctor and, sometimes, draining the fluid. If there is a mechanical injury, specific treatment is considered; if it is an inflammatory disease, the underlying disease is managed.
When to see a doctor
Seek medical care soon if you have any of these signs:
- A red, hot and very painful knee, with fever or feeling generally unwell: go to the emergency room.
- Sudden swelling after a blow, with inability to bear weight.
- The knee locks or you cannot straighten it.
- Swelling that comes back often with no clear cause.
Frequently asked questions
Does fluid on the knee go away on its own?
Sometimes, if the cause was mild irritation. But if it comes back, is large or very painful, the cause needs to be found. Leaving it unassessed can delay the diagnosis.
Is it the same as "water on the knee"?
Yes, that is the popular name for joint effusion. It can be increased synovial fluid, blood or another type depending on the cause.
Does the fluid need to be drained?
Not always. It is considered when there is a lot of fluid, it is very painful, or it needs to be analyzed to rule out infection or arthritis. The doctor decides.
Book your free assessment
Dr. Belinda Martínez reviews your case at her office in Guadalajara. The assessment is free of charge and does not commit you to any treatment.
Book a free assessmentKeep reading
This content is informational and educational; it is not a diagnosis or a treatment recommendation. If you have any symptoms, consult a health professional.