Changes in arch width during the early correction of Calss II, Division 1 malocclusions with either the Fränkel functional appliance or headgear are compared in an ongoing prospective randomized clinical trial. The data were collected from 43 children, ages 7.5 to 12.85 years, who met strict dental and cephalometric critiera for inclusion in the study. They were assigned at random to treatment with either a headgear ( n=21) or a Fränkel appliance ( n=22). Occlusal measurements included the maxillary and mandibular intermolar distances (buccal and palatal/lingual) and intercanine distances. Measurements (millimeters) were performed on casts taken every 2 months, with digital calipers accurate to 0.01 mm. Four months after the initiation of treatment, the mean maxillary intermolar distance was larger in the Fränkel group (palatal: 1.58, SE: 0.22; buccal: 1.58, SE: 0.20) than the headgear group (palatal: −0.39, SE: 0.21; buccal: 0.26, SE: 0.23), and the difference was statistically significant (palatal: p<0.0001 and buccal: p=0.0001). The mean maxillary intercanine distance increased more with the headgear (1.62, SE: 0.19) than the Fränkel appliance (0.62, SE: 0.23) p=0.003. As treatment progressed, the average intermolar distance in the headgear group increased, but was still higher in the Fränkel group by more than 1 mm. The intercanine distance remained larger in the headgear group. The mandibular intermolar and intercanine distances were higher after Fränkel therapy than with headgear. The results suggest that increases in intermolar distance relate to different mechanisms with the headgear versus Fränkel appliance, whereas the increase in intermolar distance in the Fränkel group and intercanine distance with the headgear may reflect the influence of cheek/lip muscles on tooth position.