The sudden swelling of the thigh may be related to the following reasons: 1. There may be a thrombosis of the deep vein. This situation is often seen in patients with hip trauma, such as dislocation of the hip joint, fracture between the ramus, etc., or when the patient needs to be bedridden for a long time. In this case, the local blood flow is slowed down and the blood is in a hypercoagulable state, which can easily produce thrombosis in the deep vein, followed by swelling and pain in the thigh area. 2. It may be caused by trauma to the thigh area. For example, if there is sudden swelling, the possible types of trauma are muscle contusion of the thigh, or fracture or dislocation of the thigh, etc. 3, it may be caused by skin lesions of the thigh. For example, insect bites, contact dermatitis, eczema, may cause rapid swelling of the thigh area. After the above symptoms appear, patients must go to the clinic as soon as possible to clarify the cause, and then do targeted treatment.