The purpose of this study was to assess the effects of rapid maxillary expansion on nasal airflow studies and mouth breathing habit. 15 children aged from 5 to 12 years underwent rapid palatal expansion for orthodontic abnormalities. Otolaryngologic evaluation, computerized active anterior rhinomanometric measurements and oral myo-functional disorder examination were obtained before treatment, 2 months after expansion and after a retention period of approximately 10/12 months. Based on this multidisciplinary judgment and confirmed by the rhinomanometric values two groups could be distinguished: a group of predominantly mouth breathers where the nasal airway resistance had an average decrease of 34% and a group of predominantly nasal breathers where the nasal airway resistance had an average decrease of less than 5%. From the initial 8 predominantly mouth breathers before treatment only 2 remained clinically unchanged. From these results it was concluded that for the 15 children involved in this study rapid maxillary expansion affected the nasal airway patency in a way tailored to the initial deficiency and had a significative effect on the respiratory mode.