While the foot abduction brace (FAB) plays an important role in the Ponseti method, the true function of the FAB in the treatment of idiopathic clubfoot remains unknown. In our clinical experience, we have noted that many patients with unilateral idiopathic clubfoot developed significant flatfoot in the contralateral foot during brace treatment. The purpose of this study was to investigate the natural history of the contralateral foot development during and after brace wear. We also discuss the effect of the FAB on the contralateral foot.We retrospectively reviewed 21 contralateral feet of 21 patients with unilateral idiopathic clubfoot who were treated using the Ponseti method and were conservatively followed up until the FAB was taken off (6 years of age or older). We evaluated flatfoot indicators of the contralateral foot on standing radiographs during and after brace wear and compared them against the normal reference ranges. We also evaluated the changes in the flatfoot indicators of the contralateral foot during and after brace wear.Although there was a significant difference in the flatfoot indicators between the contralateral foot and normal reference ranges during brace wear, there was no significant difference in the flatfoot indicators after brace wear. While there was no significant improvement in flatfoot indicators of the contralateral foot during brace wear, there was a significant improvement or a trend to improve after brace wear. There was no significant correlation between the contralateral flatfoot and original joint laxity.Significant flatfoot deformity was observed in the contralateral foot during brace wear. The contralateral flatfoot persisted during brace wear and improved to within normal reference ranges after brace wear. Our findings suggest that the FAB may influence the development of the contralateral foot, leading to the flatfoot.