The purpose of this study was to investigate the effects of the maxillary protractor bow appliance (MPBA) on dentoalveolar structure and skeletal morphology in patients with Class III malocclusions in different dental stages. The sample consisted of 63 treated and 57 untreated Japanese patients who all had anterior crossbites. The former group was treated with MPBA and included 34 subjects with deciduous dentition (DT group) and 29 subjects with early mixed dentition (MT group). In the DT group, MPBA was used over a mean period of 5 months to obtain positive overjet, and, in the MT group, MPBA was used over a mean period of 10 months identically. Analysis of variance with 3 factors (appliance, treatment stage, sex) between treated and untreated subjects and unpaired t tests between DT and MT groups were performed from 2 lateral cephalograms before and after treatment to compare the dentofacial changes. The mechanisms of improving anterior crossbite were similar in both groups; however, the mean skeletal and dentoalveolar changes in the DT group were significantly greater than those in the MT group. The clinical effects of MPBA treatment were greater in the deciduous-dentition group than in the early-mixed-dentition group.