Waking up with one swollen knee — while the other is perfectly fine — can be concerning, especially for older adults accustomed to joint discomfort. Unlike swelling in both knees, which often indicates systemic issues like rheumatoid arthritis, swelling in just one knee typically points to a localized problem. Recognizing the difference can help determine whether you need home care or immediate medical attention.

Common causes include injuries or overuse, which often go unnoticed but result in ligament tears, cartilage damage, or bone bruises. Swelling from injury usually occurs within hours and may be accompanied by stiffness, pain, or instability. Another frequent issue is bursitis, where inflammation of the knee’s protective sacs leads to swelling at the front of the knee, especially after prolonged kneeling or repetitive activity.

A more serious cause is joint infection (septic arthritis), marked by rapid swelling, heat, redness, fever, and severe pain. This is a medical emergency requiring urgent treatment to prevent permanent damage. Similarly, gout or pseudogout can cause intense, sudden pain and swelling in one knee due to the buildup of uric acid or calcium crystals. These attacks often occur overnight and require medication for relief.

Other causes include osteoarthritis, which may begin in one knee due to uneven wear or a past injury. Symptoms include stiffness, aching pain, and mild swelling that worsens with movement. Baker’s cysts, fluid-filled lumps behind the knee, are also common and may be related to arthritis or a cartilage tear. If the cyst bursts, swelling can extend into the calf and mimic symptoms of a blood clot, making diagnosis essential.

Finally, deep vein thrombosis (DVT), though less common, can cause swelling in the leg and knee. If accompanied by redness, tightness, or shortness of breath, it requires emergency care. If knee swelling is sudden, severe, persistent, or comes with systemic symptoms like fever or inability to walk, it’s important to consult a doctor. Swelling in one knee is often treatable — and listening to your body is the first step in protecting your long-term health and mobility.