Thirty-six patients with reflex sympathetic dystrophy primarily affecting the knee were reviewed. Injuries or operation about the patellofemoral joint triggered its onset in 64% of patients. Co-existent mechanical derangement of the knee was present in 64% of patients. Those patients who underwent sympathetic blockade or sympathectomy within one year of onset of symptoms had significantly better pain and function scores than those in whom intervention was later. Early diagnosis remains the key to successful management. Surgery for co-existent mechanical derangement in the affected knee should not be performed until the syndrome is controlled.