Introduction: Confinement measures that were imposed during the COVID-19 pandemic drastically changed the routines of the population. Some studies on the impact of confinement on glycemic control suggest a reduction of 0.1 to 0.5% in glycated hemoglobin. The objective of this study was to evaluate the impact of the COVID-19 pandemic lockdown on glycemic control in adult patients with type 1 diabetes mellitus.Methods: An observational retrospective cohort study of patients with type 1 diabetes mellitus followed in a Diabetes Unit was performed. The study compared the metabolic control of these patients before (between January 1st and March 18th, 2020) and after (between May 3rd and July 31st, 2020) the lockdown.Results: The study included 102 patients with type 1 diabetes mellitus (51% females), with a median age of 36 years (interquartile range 18.75, (24.25-43)) and a median duration of diabetes of 15 years (interquartile range 13, (8-21)). After lockdown, a significant decrease of 0.28±0.71% in glycated hemoglobin was observed (7.88±1.33% vs 7.59±1.23%, p=<0.001). In patients using continuous glucose monitoring a significant improvement in time in range was also noted (47.25±17.33% vs 49.97±18.61%, p=0.008).Conclusions: This study demonstrated an improvement in glycemic control after the lockdown. This might be explained by the positive impact of stable schedules, healthy meals and greater availability to make therapeutic adjustments to glycemic control. The fact that diabetes was considered a risk factor for the development of severe COVID-19 disease might also influence patients to increase their efforts to optimize their glycemic control.