Aims: We assessed safety and efficacy of Cambridge hybrid closed-loop (HCL) compared with usual care over 6 months in children and young people with T1D. Methods: In an open-label multicentre multinational parallel randomised controlled trial, we randomly assigned participants on insulin pump therapy aged 6 to 18 years to receive either HCL therapy (CL) or continue usual care (control) for 6 months. We used the same Cambridge model predictive control algorithm in two consecutive hardware iterations, FlorenceM then CamAPS FX. Primary endpoint was central laboratory HbA1c at 6 months. Results: We randomised 133 participants: 65 to CL and 68 to control (baseline HbA1c 8.2±0.7% vs. 8.3±0.8%). At 6 months mean HbA1c was 0.32% lower in CL compared to control (95% CI -0.59 to -0.04; p=0.02). Closed-loop usage was low (40% [26, 53]; median [IQR]) with FlorenceM due to hardware issues, and high (93% [88, 96]) with CamAPS FX. In the CamAPS FX CL group (n=21) HbA1c was 1.05% lower (95% CI -1.43 to -0.67; p Conclusion: Cambridge HCL is safe and significantly improves glycaemic control in children and young people with T1D. Efficacy relies on consistent usage of closed-loop, as demonstrated by CamAPS FX. Disclosure J. Fuchs: None. B. A. Buckingham: Advisory Panel; Self; Medtronic, Tolerion, Inc., Research Support; Self; Beta Bionics, Inc., Insulet Corporation, Medtronic, Tandem Diabetes Care. N. Davis: None. L. Dimeglio: Advisory Panel; Self; MannKind Corporation. N. Mauras: Advisory Panel; Self; Novo Nordisk, Research Support; Self; LifeScan, Medtronic, Novo Nordisk. R. Besser: None. A. Ghatak: None. S. A. Weinzimer: Advisory Panel; Self; Dompe, Zealand Pharma A/S, Speaker’s Bureau; Self; Medtronic. K. K. Hood: Consultant; Self; Cecelia Health, Cercacor, LifeScan Diabetes Institute. D. Fox: None. L. Kanapka: None. C. K. Boughton: None. C. Kollman: Consultant; Self; Diasome Pharmaceuticals, Inc., Research Support; Self; Dexcom, Inc., Tandem Diabetes Care. J. Sibayan: None. R. Beck: Consultant; Self; Bigfoot Biomedical, Inc., Diasome Pharmaceuticals, Inc., Insulet Corporation, Lilly Diabetes, vTv Therapeutics, Research Support; Self; Beta Bionics, Inc., Dexcom, Inc., Medtronic, Novo Nordisk, Tandem Diabetes Care. R. Hovorka: Research Support; Self; Abbott Laboratories, Dexcom, Inc., Medtronic, Speaker’s Bureau; Self; Dexcom, Inc., Eli Lilly and Company, Stock/Shareholder; Self; CamDiab Ltd. Dan05 consortium: n/a. J. M. Allen: None. M. E. Wilinska: Consultant; Self; CamDiab Ltd. M. Tauschmann: None. L. Denvir: None. A. Thankamony: None. F. Campbell: n/a. R. Wadwa: Consultant; Self; Tandem Diabetes Care, Research Support; Self; Dexcom, Inc., Eli Lilly and Company, Tandem Diabetes Care. Funding National Institute of Diabetes and Digestive and Kidney Diseases (UC4DK108520)