BACKGROUND Our objective was to compare the effects of 2 preadjusted appliances on angular and linear changes of the mandibular incisors, and transverse mandibular arch dimensional changes over a minimum of 30 weeks. This was a prospective, randomized, controlled, clinical trial a the Royal London Hospital, School of Dentistry, in London and the Kent and Canterbury Hospital in Canterbury, United Kingdom. METHODS Sixty- six consecutive patients satisfying the inclusion criteria were enrolled and randomly allocated to treatment with a self-ligating bracket system (SmartClip, 3M Unitek, Monrovia, Calif) and conventional preadjusted edgewise brackets (Victory, 3M Unitek). Initial study models and cephalograms were obtained within a month of starting the trial. All subjects received treatment with the following archwire sequence: 0.016-in round, 0.017 x 0.025-in rectangular, 0.019 x 0.025-in rectangular martensitic active nickel-titanium archwires, and 0.019 x 0.025-in stainless steel archwires. Final records, including study models and a lateral cephalogram, were collected a minimum of 30 weeks after initial appliance placement. Lateral cephalograms were assessed for treatment-related changes in mandibular incisor inclination and position. Transverse dimensional changes in intercanine, interpremolar, and intermolar dimensions, and the amount of crowding alleviated during the study period were assessed by comparison of pretreatment and posttreatment models. All measurements were made with a digital caliper (150 mm ISO 9001 electronic caliper, Tesa Technology, Renens, Switzerland). RESULTS Sixty patients completed the study. After adjustment for pretreatment values, duration of treatment, and amount of crowding alleviated during the study period, bracket type had little effect on incisor inclination (P = 0.437) and positional changes (P = 0.35), and intercanine (P = 0.967), inter-first premolar (P = 0.495), and inter-second premolar (P = 0.905) dimensions. However, the self-ligating appliance produced slightly more expansion in the molar region, a difference that was statistically significant (P = 0.009). Pretreatment values for incisor inclination (P = 0.044) and transverse dimensions (P = 0.000) affected inclination and transverse changes, respectively, with proclination less likely when the labial segment was proclined at the outset and expansion unlikely during leveling and alignment in wider arches. Greater alleviation of crowding during the study period resulted in more incisor proclination (P = 0.000) and advancement (P = 0.000). CONCLUSIONS There was little difference overall in the pattern of arch alignment and leveling related to the 2 preadjusted appliances. However, there was a statistically greater increase in intermolar width in the group treated with the self-ligating appliance, although the difference was only 0.91 mm.