Sudden swelling of one leg is a cause of venous thrombosis, varicose veins, phlebitis and other diseases. 1. Venous thrombosis: when venous thrombosis occurs in the lower limbs, it can be manifested as sudden swelling of unilateral lower limbs, which will be accompanied by tingling sensation, so it is necessary to consult a doctor in time and do not delay the condition, so as to avoid life-threatening accidents. 2. Varicose veins: When unilateral varicose veins are present in the leg, the affected limb will also be swollen, especially after standing for a long time, the swelling will be more obvious. 3. Phlebitis: In phlebitis, the lining of the veins is damaged, which also leads to swelling of the leg, usually accompanied by tenderness. Sudden swelling of one leg may have other reasons, when a leg is suddenly swollen, first of all, you should reduce the leg activities, reduce standing and walking, if the swelling is not relieved, it is recommended to consult a doctor in time, so as not to delay the condition.