The fragile X syndrome is one of the main etiologies of mental retardation in human beings. Some of its specific cognitive and language disturbances are nowadays well known. Disturbances of behavior are frequent. They are akin to psychotic manifestations and are mainly described in terms of autistic syndromes. The question of the possible links between autism and the fragile X syndrome is being discussed since 1980. The authors give a synthetic and critic overview of the studies reporting on the frequency of co-occurrence of these syndromes and offer some reflexion on the true questions at stakes in the debate. They emphasize the determining role of future clinical research in autism in order to objectively contribute to the progression of knowledge in this field.