The functional mandibular prognathism belong to the class III malocclusion according to the terminology of Angle. Its origins are multiple, from the abnormality of eruption of deciduous or definitive incisors to lingual dysfunction (low position of the tongue). In spite of its weak prevalence, it must be prematurely detected and treated (mixed or temporary teeth) to prevent a functional anomaly to become a skeletal anomaly. It is important at this stage to proceed to the unique gesture which allows making the differential diagnosis: it is the De Névrezé procedure; it allows obtaining a more retrusive position of the mandible to minimize the dental relations. In case of true mandibular prognathism, the maneuver does not succeed; there is no modification of the dental reports. An interceptive therapeutic phase allows finding quickly a previous correct guide and to rehabilitate the growth of jaws.