This study evaluates dental arch and alveolar widths of patients with Class II, division 1 malocclusion. Thirty female patients with Class II, division I malocclusion were compared with 30 female subjects with Class I ideal occlusion. Patients with posterior crossbites even in a single tooth were excluded from the study. According to our results, arch widths measured between maxillary second premolars and maxillary first molars were found narrower in the Class II, division 1 group, and mandibular intercanine widths were narrower in the Class I group. Interalveolar widths showed no difference between the groups. These results suggested that transverse discrepancy in Class II, division 1 patients originated from upper posterior teeth and not from the maxillary alveolar base. Therefore, slow maxillary expansion rather than rapid maxillary expansion may be considered before or during the treatment of Class Ii, division 1 patients.