Objective: To study the changes in velopharyngeal function following maxillary advancement using a rigid external distraction (RED) device. Materials and Methods: A study of 21 patients—13 male and 8 female—with maxillary dysplasia fitted with a RED device from 1999 to 2001. The average age at the time of the operation was 15.05 years. All the patients were examined preoperatively and postoperatively to determine the changes in velopharyngeal function using cephalometric analysis, perceptual speech judgment and acoustic analysis at a computerized speech lab (CSL). Results: The pharyngeal depth and the angle formed by the soft palate and the hard palate increased following maxillary distraction. The increase in rate of velopharyngeal coronary restriction (RVCR) showed deterioration of velopharyngeal closure postoperatively. A 61.9% of patients showed a significant increase in hypernasality after the operation. Deterioration in velopharyngeal closure was also proven by the findings that the energy of F0 of the vowel [i] increased significantly and the energy of consonant peak energy frequency of the fricative [s] decreased. Conclusion: Maxillary distraction has an adverse effect on velopharyngeal function and resonance, particulary for patients with cleft lip and palate.