Swollen legs and leg pain may be caused by the following: 1. The most common cause is vascular, including lower limb veins or arteries, such as arteritis, phlebitis, arterial embolism, venous embolism, these can cause impaired return and circulation, resulting in leg pain and leg swelling. 2. It may be a local infection, such as vasculitis, or other like bacterial infection, tuberculosis infection, viral infection. These infectious diseases can cause redness, swelling, heat and pain in the legs. 3. It may be a tumor, such as synovial sarcoma or osteosarcoma, which can cause circulatory disorders and even swelling and pain. 4. It may also be a rheumatoid disease, rheumatism-like disease or gout disease, which can also cause leg swelling and leg pain.