Background and aims: Mechanisms underlying the increased cardiovascular risk in type 1 diabetes (T1D) are not completely known. Hyperglycemia, hypoglycemia, and glucose variability could favor the development of atherosclerosis in T1D. Hybrid artificial pancreas (HAP) considerably improves glucose control and variability. Patients shifting from usual care to HAP represent a unique setting to test the impact of improved glucose control on vascular health. The aim of the present study was to evaluate the relation between glucose metrics and endothelial function and to assess the impact of 3-months use of HAP in patients with T1D. Materials and methods: In a prospective cohort study we enrolled patients with ≥3-year T1D duration without macrovascular complications, regularly using continuous glucose monitoring (CGM), candidates to HAP. All enrolled subjects underwent ultrasound vascular assessment including flow mediated dilation (FMD), arterial stiffness, and intima-media thickness (IMT) before and after 3-month use of HAP. Time in range (TIR) 70-180 mg/dl, time above range (TAR) 181-250 mg/dl and >250 mg/dl, time below range (TBR) 69-54 mg/dl and <54 mg/dl, coefficient of variation (CV) as evaluated by CGM were recorded. Results: A total of 24 T1D individuals (40% males, mean age 40±18 years) were enrolled in the study. Nine patients (37%) were on multiple daily injections (basal-bolus) and 15 (63%) on insulin-pump (no automated systems). At baseline, FMD of brachial artery was 5.4±3.5%, arterial stiffness was 5.5±1.6 m/s, and IMT 0,6±0.2 mm. A significant inverse correlation was found between FMD values and TAR 181-250 mg/dl (rho=-0.538, p=0,007). The assessment after 3-month HAP use (n=13) showed a significant increase in TIR (from 52±12.4 to 72±17%, p<0.001) and a significant reduction in TAR 181-250 mg/dl (from 30±14 to 19±11%, p=0.005) and in glucose variability (from 36±6% to 32±4.3, p=0.019). A significant improvement in FMD values (from 5.42±3.5 to 9.35±3.5%, p<0.001) was observed after 3-month HAP while arterial stiffness and IMT remained substantially unchanged. The 3-month changes in FMD inversely correlated with changes in TAR 181-250 mg/dl (rho=-0.566, p=0.045). Conclusion: In individuals with T1D vascular function was inversely associated with TAR. The use of HAP for 3 months induced a significant improvement in endothelial function associated with improvement in TAR.