This study evaluated the effects of rapid palatal expansion (RPE) on nasopharyngeal airway size, head posture, and cervical curvature angle in children with nasal obstruction. The patients were 45 female subjects (8-15 years of age) who had a reduced nasopharyngeal airway size and were subjectively assessed as being mouth breathers and requiring palatal expansion. They were randomly allocated to one of two groups: 23 subjects in the first group were treated with RPE, while the 22 subjects in the other group were monitored for approximately 14 months prior to commencing therapy, and became untreated controls. Lateral skull radiographs, taken in the natural head position, were obtained at the first visit (T0) and 6 (T1) and 12 (T2) months later for all subjects. The differences between the cephalometric variables at baseline and after 6 and 12 months were evaluated with a one-way repeated measures analysis of variance. Where significant interactions were found, a Bonferroni corrected paired Student's t-test was performed for pairwise comparisons. Changes in cephalometric variables within the experimental groups were tested by paired Student's t-tests as a post hoc procedure. Finally, a correlation matrix, using the Pearson correlation coefficient, was computed in order to evaluate the relationship between the change in airway adequacy and (1) the amount of maxillary expansion, (2) chronological age, (3) the amount of time that the appliance was activated, and (4) morphological and postural measurements of the face. At T1, children under active treatment showed a statistically significant increase in nasopharyngeal airway size, cervical curvature angle, and flexion of the head, together with a significant decrease in craniocervical angulation (all P < 0.05). These changes were all found to be stable at T2. No significant changes were seen in the control group. The correlation coefficients indicated a significant correlation between nasopharyngeal airway size and craniocervical angulation (SN/OPT angle; r = -0.61, P < 0.05). The findings indicate that RPE is capable of increasing nasopharyngeal airway size in young females, which results in a decrease in craniocervical angulation. Clinically, the findings seem to suggest that improvement of respiratory function could result in a change in head posture.