Leg Swelling: What Causes It 🩸 Swollen legs can be caused by various factors, such as prolonged standing or sitting, varicose veins, lymphatic system issues, or even heart and kidney diseases. If your leg suddenly swells without any apparent reason, it's advisable to seek medical attention promptly to rule out any underlying health risks. Regular exercise, avoiding long periods of sitting or standing, and wearing comfortable shoes can help alleviate the symptoms of leg swelling.