In this study 23 oral breathing children suffering from maxillary hypoplasia (endognathia associated with skeletal class II or III), selected for rapid maxillary expansion (RME) have been investigated by active anterior rhinometry. None of these patients presented O.R.L. pathologies during clinical examination except for some sporadic cases of adenoid hypertrophy (5 cases). Rhinomanometric and cephalometric examinations carried out before and after RME treatment showed a good correlation between the nasal respiratory function parameters and the structural cephalometric elements investigated by means of teleradiography. In particular, an important reduction in nasal respiratory resistance in all patients with conversion from oral to nasal respiration in the majority of cases corresponds, together with cross-bite resolution, to increased transversal dimension of the maxilla produced by RME. A clear regression in adenoid hypertrophy, where present, is also noted without any type of O.R.L. treatment. The improved respiratory situation could therefore produce benefits on the trophism of the nasal mucous and the lymphatic rhinopharyngeal tissue. Agreement between clinical, radiological and rhinomanometric findings confirm the usefulness of this method in diagnosing and following-up patients with this problem.