A sudden swelling above the knee may be caused by trauma, infection, tumor and tumor-like lesions and other diseases. 1. Trauma: due to the external force on the knee caused by local capillary rupture bleeding and oozing, the patient may appear above the knee suddenly swollen a bag. 2. Infection: due to bacteria, fungi and other pathogens infecting the local area, causing local inflammation, oozing and other pathological changes, patients may have a sudden swelling above the knee. 3. Tumor and tumor-like lesions: benign lipoma and fibroma, malignant soft tissue sarcoma, synovial cysts, tendon sheath cysts and other diseases can lead to a sudden swelling above the knee. Sudden swelling of a bag above the knee may also have other causes, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment.