OBJECTIVE: We aimed to assess the feasibility and safety of hybrid closed-loop insulin delivery in children with type 1 diabetes aged 1-7 years as well as evaluate the role of diluted insulin on glucose control. RESEARCH DESIGN AND METHODS: In an open-label, multicenter, multinational, randomized crossover study, 24 children with type 1 diabetes on insulin pump therapy (median age 5 years [interquartile range 3-6] and mean ± SD HbA1c 7.4 ± 0.7% [57 ± 8 mmol/mol] and total insulin 13.2 ± 4.8 units/day) underwent two 21-day periods of unrestricted living and we compared hybrid closed-loop with diluted insulin (U20) and hybrid closed-loop with standard strength insulin (U100) in random order. During both interventions, the Cambridge model predictive control algorithm was used. RESULTS: The proportion of time that sensor glucose was in the target range between 3.9 and 10 mmol/L (primary end point) was not different between interventions (mean ± SD 72 ± 8% vs. 70 ± 7% for closed-loop with diluted insulin vs. closed-loop with standard insulin, respectively; P = 0.16). There was no difference in mean glucose levels (8.0 ± 0.8 vs. 8.2 ± 0.6 mmol/L; P = 0.14), glucose variability (SD of sensor glucose 3.1 ± 0.5 vs. 3.2 ± 0.4 mmol/L; P = 0.16), or the proportion of time spent with sensor glucose <3.9 mmol/L (4.5 ± 1.7% vs. 4.7 ± 1.4%; P = 0.47) or <2.8 mmol/L (0.6 ± 0.5% vs. 0.6 ± 0.4%; P > 0.99). Total daily insulin delivery did not differ (17.3 ± 5.6 vs. 18.9 ± 6.9 units/day; P = 0.07). No closed-loop-related severe hypoglycemia or ketoacidosis occurred. CONCLUSIONS: Unrestricted home use of day-and-night closed-loop in very young children with type 1 diabetes is feasible and safe. The use of diluted insulin during closed-loop does not provide additional benefits compared with standard strength insulin.