Sixty-one postorthodontic patients were examined to determine the effect of rapid and slow palatal expansion therapy on the periodontal supporting structures located at the buccal aspects on the maxillary first permanent molars. Twenty-eight patients were treated with a rapid maxillary expansion (RME) technique using a tissue-borne, fixed, split acrylic appliance (Fig. 1), while thirty-three underwent slow palatal expansion with a quad-helix appliance (Fig. 3). The two groups demonstrated mean increases in maxillary first molar width of 4.6 mm. and 4.3 mm., respectively. Twenty-eight patients treated with a similar light-wire edgewise technique but no palatal expansion were used as a control. Four periodontal parameters were examined, including level of marginal alveolar bone, attachment levels (from CEJ), probing depths, and width of keratinized gingiva. All measurements were made by the same examiner and recorded to the nearest 0.5 mm., using a specially machined and calibrated periodontal probe. The results demonstrated tht both lateral expansion groups exhibited minimal differences in periodontal condition when compared to the control group. Periodontal status was good in all groups, with average attachment levels being 0.65 mm., 0.36 mm., and 0.51 mm. in the rapid expansion, slow expansion, and control groups, respectively. Although the mean differences were clinically small, individual variation was evident (Fig. 5). Among the few persons who exhibited the more marked periodontal breakdown at the central aspect of the first molars, most were found in the RME group. Except for the type of expansion therapy experienced, none of a number of other orthodontic variables examined demonstrated a statistically significant influence on the periodontal tissues.