Lymphedema is swelling, usually in extremity, due to a blockage or abnormality of the lymph drainage system. It is usually irreversible and can be quite disabling. Lymphedema can be caused by surgery to remove lymph nodes. In the past, breast cancer patients often underwent removal of most of the lymph nodes in the underarm area as part of their treatment; 40% of these women developed lymphedema of the arm on that side. Since the development of the sentinel node biopsy, a procedure in which only one or a few lymph nodes are removed, only about 5% of patients develop arm swelling.