The Fibromialgia Syndrome (FS) is a common clinical entity which may produce symtoms and signs related to multiple fields of Medicine. Typical clinical characteristics of FS include extensive pain, presence of sensitive points during exploration, morning stiffness, asthenia and non-refresing sleep. Frequently, associated rheumatologic diseases are observed, as rheumatoid arthritis, osteoarthrosis and vertebral disorders. In FS, complementary tests are usually normal. The most widely accepted hypothesis suggests that this is a disorder affecting modulation of pain sensitivity.