Fibromyalgia is a chronic pain syndrome of which there remains difficult to evaluate the functional repercussion. As in other affections without, or with weak shown organic substrate, the evaluation of the work capacity requires a rigorous approach, which adopts an integrative bio-psycho-social model. Diagnostic classification poses problems insofar as the same syndromic association can result in different diagnoses. The object of this article is to highlight some difficulties related to the coexistence of medical and legal logics which can sometimes be divergent. Generalization, on the legal and administrative side, of simple dichotomic rules, must in no manner lead the physician to make the saving of taking into account the complexity.