RATIONALE: Elexacaftor/tezacaftor/ivacaftor (ELX/TEZ/IVA) was shown to be efficacious and safe in patients aged 12 years and older with cystic fibrosis and at least one F508del-CFTR allele, but has not been evaluated in children <12 years of age. OBJECTIVES: To assess the safety, pharmacokinetics, and efficacy of ELX/TEZ/IVA in children 6 through 11 years of age with F508del-minimal function or F508del-F508del genotypes. METHODS: In this 24-week open-label Phase 3 study, children (N=66) weighing <30 kg received 50% of the ELX/TEZ/IVA adult daily dose (ELX 100 mg once daily, TEZ 50 mg once daily, and IVA 75 mg every 12 hours) while children weighing ≥30 kg received the full adult daily dose (ELX 200 mg once daily, TEZ 100 mg once daily, and IVA 150 mg every 12 hours). MEASUREMENTS AND MAIN RESULTS: The primary endpoint was safety and tolerability. The safety and pharmacokinetic profiles of ELX/TEZ/IVA were generally consistent with those observed in older patients. The most common reported adverse events (AEs) included cough, headache, and pyrexia; in most of the children who had AEs, these were mild or moderate in severity. Through Week 24, ELX/TEZ/IVA treatment improved the ppFEV1 (10.2 percentage points; 95% confidence interval [CI], 7.9 to 12.6), Cystic Fibrosis Questionnaire-Revised respiratory domain score (7.0 points; 95% CI, 4.7 to 9.2), lung clearance index2.5 (-1.71 units; 95% CI, -2.11 to -1.30), and sweat chloride (-60.9 mmol/L; 95% CI, -63.7 to -58.2); body mass index-for-age z-score increased over the 24-week treatment period compared to the pre-treatment baseline. CONCLUSIONS: Our results show ELX/TEZ/IVA is safe and efficacious in children 6 through 11 years of age with at least one F508del-CFTR allele, supporting its use in this patient population. Clinical trial registration available at www.clinicaltrials.gov, ID: NCT03691779 This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/).