Dercum's disease (adiposis dolorosa, lipomatosis dolorosa morbus Dercum), is a rare disorder resulting in painful fatty deposits around the upper legs, trunk, and upper arms. The portrait painted of Dercum's disease is very complicated, with many other disorders seen associated with the disease. There are no clear pathological mechanisms known, although it is suspected that there is either a metabolic or autoimmune component involved. Here, the authors review the literature to date, including some information from their own studies. In particular, the authors will look at the different strands of evidence pointing to the pathological mechanism of the disorder.