Reflex sympathetic dystrophy (RSD) is a seldom-considered diagnosis in knee pain of undetermined etiology. However, in a retrospective study of 67 patients with unexplained knee pain, 14 patients met criteria to establish the diagnosis of reflex sympathetic dystrophy. These patients' clinical profiles, physical findings, and laboratory tests and treatments support the diagnosis of reflex sympathetic dystrophy of the knee and provide guidelines for treatment of the condition.