The short answer
A pleural effusion is a buildup of fluid between the thin layers of tissue that line the lung and the chest wall. MedlinePlus lists heart failure as the most common cause of the transudative type. Infection, inflammation and cancer can also cause it. A sample of the fluid can help find the cause.
A pleural effusion is fluid between the lung and the chest wall.
MedlinePlus lists heart failure as the most common cause of the transudative type. Infection, inflammation, and tumors can also cause it.
A needle test called thoracentesis can sample the fluid. Get care right away for shortness of breath. Call 911 for severe trouble breathing.
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This page is for people whose chest x-ray, CT, or ultrasound report mentions a "pleural effusion." It explains what the phrase means, common causes, and what to ask your care team.
What pleural effusion means
The pleura are thin layers of tissue. One layer covers the lungs, and one lines the inside of the chest wall. A pleural effusion is a buildup of fluid between those layers.
It can show up on a chest x-ray, CT scan, or ultrasound.
Two main types
MedlinePlus describes two types:
- Transudative: fluid leaks into the space because of pressure in blood vessels or low protein in the blood. Heart failure is the most common cause.
- Exudative: caused by blocked blood or lymph vessels, inflammation, infection, lung injury, or tumors.
How the cause is found
A procedure called thoracentesis uses a needle to take a sample of the fluid. The fluid can be checked for infection, cancer cells, protein, and cell counts. Blood tests or a heart ultrasound may also be done.
Treatment depends on the cause. If you already have cancer and fluid keeps coming back, see Malignant Pleural Effusion.
Symptoms and when to get care
Some effusions cause no symptoms. Others can cause chest pain that is worse with coughing or deep breaths, shortness of breath, fast breathing, cough, fever, or hiccups.
MedlinePlus advises contacting your provider or going to the emergency room if you have symptoms, or shortness of breath right after thoracentesis. For severe trouble breathing, call 911.
Ask these before you leave
- How large is the effusion? Is it on one side or both?
- What is the likely cause?
- Do I need thoracentesis or other tests?
- Which symptoms should I report right away?
Related pages
See Malignant Pleural Effusion, What Does Atelectasis Mean on a Chest X-Ray or CT?, and What Does Ascites Mean on a Scan?
Sources
Common questions
Does a pleural effusion mean cancer?
Not necessarily. MedlinePlus lists heart failure as the most common cause of the transudative type. Infection, inflammation and tumors are other causes. Testing the fluid can help show the cause.
What is thoracentesis?
A procedure that uses a needle to take out some fluid from around the lung. The fluid can be checked for infection, cancer cells, protein and cell counts.
What does a small effusion mean?
It means a small amount of fluid. Some effusions cause no symptoms. Ask your care team whether yours needs testing or follow-up.
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Sources last checked: 2026-09-27 what this meansLast updated: 2026-09-27Next planned review: 2027-09-27
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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