Aim: To evaluate, via occlusal radiographs, transversal alterations that result from rapid maxillary expansion with the Hyrax appliance. Methods: The sample consisted of 14 children of both genders, in mixed dentition, with unilateral or bilateral posterior crossbite. Occlusal radiographs were taken at the beginning and at the end of maxillary expansion. The following variables were measured: intermolar distance, interincisal distance, incisor interapex distance, interbase distance, interarm distance, and interincisor inclination. The interapex and interbase distances had their radio-graphic amplification corrected. Results: A mean opening of 7.65 mm of the expansion screw resulted in an increase of the intermolar distance of 7.40 mm, progressively smaller increases of the interarm distance of the appliance of 6.6 mm, and of the incisor interapex distance of 5.5 mm. The distance between the maxillary incisors increased 1.9 mm, which indicated more posterior than anterior expansion. In addition, some buccal inclination of the molars was found, but the apexes of the incisors opened 1.9 mm less than the intermolar distance. Incisor inclination changed 9.7 degrees as a result of the rapid maxillary expansion therapy. Conclusions: Analyzing the occlusal radiographs, the dental arch showed more posterior than anterior opening and a change of 9.72 degrees in incisor angulation as a result of rapid maxillary expansion. World J Orthod 2007; 8: 277–284.