Aim: To evaluate the effects of maxillary expansion treatment on the nasopharyngeal area in the sagittal plane through the use of lateral cephalograms and to determine the relationship between these changes and those in the maxilla. Methods: Expansion treatment was used in 20 patients; 10 were treated with a bonded Mini-expander appliance (slow maxillary expansion) and the other 10 were treated with a bonded Hyrax appliance (rapid maxillary expansion). The active treatment periods were 3.28 ± 0.97 months and 1.08 ± 0.24 months, respectively. Lateral cephalograms and hand-wrist radiographs were taken at the beginning and at the end of active treatment. Thirteen nasopharyngeal and 10 maxillary parameters were evaluated on the lateral cephalograms, and the nasopharyngeal airway was measured with a digital planimeter. Analysis of variance and paired t tests were applied. The relationship between the nasopharyngeal and skeletal parameters was evaluated with the Pearson correlation analysis. Results: The nasopharyngeal height, depth of the airway, and bony nasopharyngeal space increased; the decrease in the thickness of the adenoids was statistically significant. The statistically significant increase in the nasopharyngeal area emphasized the effect of expansion on the nasopharyngeal area. Conclusion: Although significant increases in the sagittal nasopharyngeal depth and bony area were found, these changes did not correlate with movement of the maxilla. The significant increase in the nasopharyngeal height was correlated with movement of the maxilla. World J Orthod 2002;3:211–216.