INTRODUCTION: In this retrospective study, we examined and compared the pretreatment and posttreatment arch widths and buccal corridor changes in subjects who had received orthodontic treatment either with or without premolar extractions. METHODS: Pretreatment and posttreatment casts, frontal smiling photographs, and lateral cephalograms of 30 extraction and 27 nonextraction patients were analyzed to determine any significant differences in arch widths, buccopalatal inclinations of the teeth, and buccal corridor widths and areas, both within and between the 2 groups. Relationships between buccal corridor measurements and corresponding arch widths and buccopalatal inclinations of the teeth were also examined. RESULTS: There was a significant increase in the posttreatment maxillary intercanine width in the extraction group but not in the nonextraction group. Both the pretreatment and posttreatment arch widths between the maxillary first molars and at the level of the posterior rugae were greater in the nonextraction group than in the extraction group. There were no significant differences in any buccal corridor widths or areas measured between the extraction and nonextraction subjects. CONCLUSIONS: Because they reflect different morphologies, there are likely to be significant differences in average maxillary anterior and posterior posttreatment arch widths in those treated with or without extractions. However, these arch width differences are not likely to be discernible in extraction and nonextraction patients as differences in the buccal corridor widths and areas.