Background: Lower incisor proclination has been shown to increase during Class II correction with appliances such as the Forsus. This lower incisor flaring shows great variability among patients. The purpose of this study was to evaluate the effects of Class II severity, mandibular growth, and occlusal plane rotation on lower incisor flaring in patients treated with the Forsus appliance. Materials and Methods: 121 records of Class II patients treated with the Forsus appliance were analyzed retrospectively. Cephalograms were traced at three time points. ANOVA was used to compare changes in cephalometric measurements over time. Pearson’s correlation was used to test for relationships between variables. Multiple regression tested for correlation between multiple variables. Results/Conclusions: Lower incisor flaring during Class II correction with the Forsus appliance was only weakly associated with sex, molar relationship change, and occlusal plane steepening. There was no correlation between the change in lower incisor proclination and mandibular growth.