The aim of this retrospective study was to examine the dental arch width changes of extraction and nonextraction treatment in Class I patients. The study was performed on pretreatment and posttreatment dental casts of 60 patients (30 extraction and 30 nonextraction). The mean ages were 14.3 +/- 2.02 years for the extraction group and 14.1 +/- 2.9 years for the nonextraction group. The maxillary and mandibular crowding was -6.7 +/- 3.1 and -6.3 +/- 2.8 mm for the extraction group and -4.5 +/- 3.6 and -2.1 +/- 3.5 mm for the nonextraction group, respectively. The intercanine and intermolar arch width measurements were measured using a digital caliper. Paired samples t-test was used to evaluate the treatment changes within each group. To compare the changes between groups, independent samples t-test was performed. At the start of treatment, the maxillary and the mandibular intercanine and intermolar widths of both groups did not differ statistically. At the end of treatment, maxillary and mandibular intercanine widths of both groups increased significantly. The mandibular intermolar width decreased significantly for the extraction group and the maxillary intermolar width increased significantly for the nonextraction group. The decrease in maxillary intermolar width for the extraction group and the increase in mandibular intermolar width for the nonextraction group were not significantly different. No differences were observed between the groups in maxillary and mandibular intercanine widths. Maxillary and mandibular intermolar width indicated a significantly larger value in the nonextraction group than that in the extraction group.