Common causes of sudden leg swelling: First, deep vein thrombosis of the lower extremity can lead to sudden swelling of one side of the leg, and deep vein thrombosis of the lower extremity is common in patients who are bedridden for a long time, or patients with fractures, because the blood flow is slow and blood stagnation can form thrombosis, at this time, deep vein ultrasound can be performed to further clarify the diagnosis, whether there is thrombosis or not. Secondly, sudden leg swelling is often seen in patients with traumatic crush or fracture, such as soft tissue contusion due to trauma and leg swelling due to bleeding and inflammation after fracture, which can be diagnosed by X-ray or MRI. Third, the common cause of leg swelling may be localized lymphangitis, which may lead to leg swelling and increased skin temperature, fever, and pain.