Reflex sympathetic dystrophy (RSD) has been described primarily in the upper extremity and is infrequently considered part of the differential diagnosis of postamputation pain. The manifestations of autonomic dysfunction may mimic other potential diagnoses of postoperative stump pain. We report a 47-year-old man who developed RSD of the knee during the mobilization phase after below-knee amputation for atherosclerotic peripheral vascular disease. The diagnosis was made by clinical examination, radiography, and scintigraphy without the need for any invasive studies. The characteristic diagnostic findings for the knee during the acute phase are emphasized and the therapy described.