Down syndrome (DS) is the most common aneuploid disorder at birth. The life expectancy of persons with DS has improved over the last forty years and is now at about sixty years. Phenotypic characteristics include general hypotonia, maxillary hypoplasia with a small oral cavity and a somewhat larger appearing tongue, frequent constricted maxillary arch, nasal obstruction and others. This prospective study assesses the effects of rapid maxillary expansion (RME) on nasal patency of children with DS, using acoustic rhinometry (AR). Twenty four children with DS, aged 5 to 12 years, had been randomly allocated to the RME and control groups. AR was performed to these individuals prior to expansion, approximately one month after, post maximal expansion, and after a 5 months period of retention. The data between the two groups were compared. Rapid maxillary expansion produced a significant augmentation of nasal volume in children who had been treated (p< 0.05) compared to the control group; these results were stable through the period of retention.