BACKGROUND: In phase 2 trials in people with cystic fibrosis aged 18 years and older, vanzacaftor-tezacaftor-deutivacaftor has been shown to be a safe and effective, once-daily cystic fibrosis transmembrane conductance regulator (CFTR) modulator. Restoring normal CFTR function early in life has the potential to prevent manifestations of cystic fibrosis. We aimed to evaluate the safety, tolerability, efficacy, and pharmacokinetics of vanzacaftor-tezacaftor-deutivacaftor in children with cystic fibrosis aged 6-11 years. METHODS: In this multicentre, single-arm, phase 3 trial (RIDGELINE Trial VX21-121-105), participants were enrolled across 33 clinical sites that care for children with cystic fibrosis in eight countries (Australia, France, Germany, Netherlands, Sweden, Switzerland, the UK, and the USA). Eligible participants were aged 6-11 years with at least one elexacaftor-tezacaftor-ivacaftor-responsive CFTR variant, FEV(1) % predicted of 60% or higher, and stable cystic fibrosis as determined by investigators. Before study treatment, participants were either on stable elexacaftor-tezacaftor-ivacaftor for at least 28 days before screening or received the combination for a 4-week run-in period. Participants then received vanzacaftor-tezacaftor-deutivacaftor (<40 kg bodyweight: vanzacaftor 12 mg, tezacaftor 60 mg, and deutivacaftor 150 mg orally as three fixed-dose combination tablets once daily;