Background: Current systemic treatments for children younger than 6 years with moderate-to-severe atopic dermatitis that is uncontrolled with topical therapies might have suboptimal efficacy and safety. Dupilumab is approved for older children and adults with atopic dermatitis and for other type 2 inflammatory conditions. We aimed to evaluate efficacy and safety of dupilumab with concomitant low-potency topical corticosteroids in children aged 6 months to younger than 6 years with moderate-to-severe atopic dermatitis., Methods: This randomised, double-blind, placebo-controlled, parallel-group, phase 3 trial was conducted in 31 hospitals, clinics, and academic institutions in Europe and North America. Eligible patients were aged 6 months to younger than 6 years, with moderate-to-severe atopic dermatitis (Investigator's Global Assessment [IGA] score 3-4) diagnosed according to consensus criteria of the American Academy of Dermatology, and an inadequate response to topical corticosteroids. Patients were randomly assigned (1:1) to subcutaneous placebo or dupilumab (bodyweight ≥5 kg to <15 kg: 200 mg; bodyweight ≥15 kg to <30 kg: 300 mg) every 4 weeks plus low-potency topical corticosteroids (hydrocortisone acetate 1% cream) for 16 weeks. Randomisation was stratified by age, baseline bodyweight, and region. Patient allocation was done via a central interactive web response system, and treatment allocation was masked. The primary endpoint at week 16 was the proportion of patients with IGA score 0-1 (clear or almost clear skin). The key secondary endpoint (coprimary endpoint for the EU and EU reference market) at week 16 was the proportion of patients with at least a 75% improvement from baseline in Eczema Area and Severity Index (EASI-75). Primary analyses were done in the full analysis set (ie, all randomly assigned patients, as randomly assigned) and safety analyses were done in all patients who received any study drug. This study was registered with ClinicalTrials.gov, NCT03346434., Findings: Between June 30, 2020, and Feb 12, 2021, 197 patients were screened for eligibility, 162 of whom were randomly assigned to receive dupilumab (n=83) or placebo (n=79) plus topical corticosteroids. At week 16, significantly more patients in the dupilumab group than in the placebo group had IGA 0-1 (23 [28%] vs three [4%], difference 24% [95% CI 13-34]; p<0·0001) and EASI-75 (44 [53%] vs eight [11%], difference 42% [95% CI 29-55]; p<0·0001). Overall prevalence of adverse events was similar in the dupilumab group (53 [64%] of 83 patients) and placebo group (58 [74%] of 78 patients). Conjunctivitis incidence was higher in the dupilumab group (four [5%]) than the placebo group (none). No dupilumab-related adverse events were serious or led to treatment discontinuation., Interpretation: Dupilumab significantly improved atopic dermatitis signs and symptoms versus placebo in children younger than 6 years. Dupilumab was well tolerated and showed an acceptable safety profile, similar to results in older children and adults., Funding: Sanofi and Regeneron Pharmaceuticals., Competing Interests: Declaration of interests ASP has been an investigator for AbbVie, AnaptysBio, Dermavant, Eli Lilly, Incyte, Janssen, Krystal Biotech, LEO Pharma, Regeneron Pharmaceuticals, and UCB; a consultant with honorarium for AbbVie, Acrotech, Almirall, Amgen, Amryt Pharma, Arcutis Antiobix, Arena Pharmaceuticals, Azitra, BioCryst, BiomX, Boehringer Ingelheim, Botanix, BridgeBio, Bristol Myers Squibb, Castle Creek Biosciences, Catawba Research, Eli Lilly, Exicure, Gilead, Incyte, Janssen, Johnson & Johnson, Kamari Pharma, LEO Pharma, Novartis, OM Pharma, Pfizer, Pierre Fabre Dermo-Cosmetics, RAPT Therapeutics, Regeneron Pharmaceuticals, Sanofi, Seanergy, UCB, and Union; and on the data and safety monitoring board for AbbVie, Abeona, Bausch, Galderma, and Novan. ELS has been an investigator for AbbVie, Eli Lilly, Incyte, Kyowa Hakko Kirin, LEO Pharma, Pfizer, Regeneron Pharmaceuticals, Sanofi, and Trevi Therapeutics; received consultant fees from AbbVie, Amgen, Arena Pharmaceuticals, Aslan Pharma, Benevolent AI Bio Limited, BiomX, Bluefin Biomedicine, Boehringer Ingelheim, Boston Consulting Group, Collective Acumen, Coronado, Corevita, Dermavant, Eli Lilly, Evidera, ExcerptaMedica, Forté Bio RX, Galderma, GSK, Incyte, Janssen, Kyowa Hakko Kirin, LEO Pharma, Menlo Therapeutics, Merck, Novartis, Pfizer, Pierre Fabre Dermo-Cosmetics, Regeneron Pharmaceuticals, Roivant, Sanofi, SPARC India, Trevi Therapeutics, and Valeant; received study grants from AbbVie, Amgen, Arcutis, Aslan Pharma, Corevita, Eli Lilly, Incyte, Kyowa Hakko Kirin, LEO Pharma, Novartis, Pfizer, Regeneron Pharmaceuticals, Sanofi, and Trevi Therapeutics; served as a speaker for Eli Lilly, Incyte, LEO Pharma, Pfizer, Regeneron Pharmaceuticals, and Sanofi; and served on advisory boards for Arena Pharmaceuticals, Eli Lilly, GSK, Janssen, Kyowa Hakko Kirin, LEO Pharma, Pfizer, Regeneron Pharmaceuticals, and Sanofi. ECS has been a consultant for AbbVie, Dermavant, Eli Lilly, Pfizer, Regeneron Pharmaceuticals, and Verrica Pharmaceuticals; on the data and safety monitoring board for GSK, LEO Pharma, Novan, Pfizer, and USB; served on advisory boards for Sanofi; and a principal investigator in clinical trials for Eli Lilly, Regeneron Pharmaceuticals, and Verrica Pharmaceuticals. MJC has been an investigator and consultant for Astellas, Galapagos, Hyphens Pharma, Johnson & Johnson, LEO Pharma, L’Oréal, Novartis, Oxagen, Pfizer, Reckitt Benckiser, Regeneron Pharmaceuticals, and Sanofi; and a consultant for AbbVie, Almirall, Anacor Pharmaceuticals, Boots, Dermavent, Galderma, Menlo Therapeutics, and Proctor & Gamble; and received research grants from Hyphens Pharma, Johnson & Johnson, LEO Pharma, L’Oréal, Pfizer, Regeneron Pharmaceuticals, and Sanofi. AW has been an investigator for Beiersdorf, Eli Lilly, Galderma, LEO Pharma, MedImmune, Novartis, Pfizer, Regeneron Pharmaceuticals, and Sanofi; a consultant for AbbVie, Almirall, Anacor Pharmaceuticals, Eli Lilly, Galapagos, Galderma, LEO Pharma, MedImmune, Novartis, Pfizer, Regeneron Pharmaceuticals, and Sanofi; and received research grants from Beiersdorf, LEO Pharma, and Pierre Fabre. PDA has been an investigator for Regeneron Pharmaceuticals; and received a research grant and been an advisor for Sanofi. WS has been a speaker, advisory board member, and investigator for AbbVie, Amgen, AstraZeneca, GSK, LEO Pharma, Pfizer, Regeneron Pharmaceuticals, and Sanofi; a consultant for AbbVie, LEO Pharma, and Regeneron Pharmaceuticals; received investigator grants from AbbVie, Aimmune Therapeutics, Genentech, GSK, Incyte, LEO Pharma, Novartis, Pfizer, Teva, and Vanda Pharmaceuticals; a speaker for Teva; and an advisor for Genentech. MEG has been an investigator for AbbVie, Arcutis Biotherapeutics, Dermira, Dermavant, Eli Lilly, Incyte, Krystal Biotech, Regeneron Pharmaceuticals, Sun Pharma, and Verrica Pharmaceuticals; a speaker for Galderma, Pfizer, Primus Pharmaceuticals, Regeneron Pharmaceuticals, and Sanofi; and a consultant for Unilever and Verrica Pharmaceuticals. LCS has been an investigator for DBV Technologies and Regeneron Pharmaceuticals; received research support from Genentech; and has been a consultant for AbbVie, Alladapt Immunotherapeutics, LEO Pharma, Regeneron Pharmaceuticals, and Sanofi. RS has been an investigator for Galderma, Regeneron Pharmaceuticals, and UCB; an advisory board member for LEO Pharma and Pfizer; and speaker for Beiersdorf. BL has been an investigator for Castle, Dermira, Franklin Biosciences, and Pfizer; an investigator, speaker, and consultant for AbbVie, Dermtech, Eli Lilly, Incyte, LEO Pharma, Regeneron Pharmaceuticals, and UCB; an investigator and consultant for Strata; and a speaker and consultant for Dermavant and Sanofi. SM has been an investigator for AstraZeneca, Pfizer, and Regeneron Pharmaceuticals. ZW, YS, BA, MD, MPK, MAK, DMW, GDY, and AB are employees and shareholders of Regeneron Pharmaceuticals. LPM, EL, NP, AD-B, and JTO are employees of and may hold stock or stock options in Sanofi. NA is a former employee and shareholder of Regeneron Pharmaceuticals., (Copyright © 2022 Elsevier Ltd. All rights reserved.)