Achondroplasia (ACH) is a monogenic skeletal dysplasia caused by gain-of-function variants in FGFR3, leading to impaired endochondral ossification and foramen magnum stenosis. Although vosoritide promotes endochondral ossification, its effects on foramen magnum development remain unclear. In this study, we evaluated the impact of early vosoritide administration on foramen magnum development in infants and young children with ACH, using longitudinal magnetic resonance imaging.