Objective: To determine by low-dose computed tomography (CT) protocol the dental and periodontal effects of rapid maxillary expansion (RME). Materials and Methods: The sample comprised 17 subjects (7 males and 10 females), with a mean age at first observation of 11.2 years. Each patient underwent expansion of 7 mm. Multislice CT scans were taken before rapid palatal expansion (TO), at the end of the active expansion phase (T1), and after a retention period of 6 months (T2). On scanned images, measurements were performed at the dental and periodontal levels. Mean differences between measurements at TO, T1, and T2 were examined through analysis of variance (ANOVA) for repeated measures with post-hoc tests. Results: All interdental transverse measurements were significantly increased at both T1 and T2 with respect to TO. In the evaluation of T0-T1 changes, periodontal measurements were significant on the buccal aspect of banded teeth with a reduction in alveolar bone thickness corresponding to the mesial (-0.5 mm; P < .05) and distal (-0.4 mm; P < .05) roots of the right first molar and to the mesial root of the left first molar (-0.3 mm; P < .05). In the evaluation of overall T0-T2 changes, the lingual bone plate thickness of both first molars was found to be significantly increased (+0.6 mm; P < .05). Conclusions: RME therapy induces a significant increase in the transverse dimension of the maxillary arch in growing subjects without causing permanent injury to the periodontal bony support of anchoring teeth discernible on CT imaging. (Angle Orthod. 2009;79:24-29.)