Aims: We aimed to estimate effects of insulin pump therapy (IPT) on HbA1c level, HbA1c variability, and risk of hospitalised diabetic ketoacidosis (DKA) and severe hypoglycaemia (SH), compared with multiple daily insulin injections (MDI).Methods: We identified a cohort of all adults with type 1 diabetes in Denmark using national registry data and assigned each individual to either IPT (treatment) or MDI (control) from 2010 to 2020. We estimated average treatment effects on the treated (ATT) and treatment effects among population subgroups using treatment-staggered difference-in-differences.Results: The cohort consisted of 26,687 individuals with a collective 243,601 person-years of observation; 38,823 (16 %) were IPT person-years. We identified an ATT for HbA1c of-0.33 % (95 % CI-0.39 to-0.27;-3.6 mmol/ mol [95 % CI-4.2 to-2.9]). ATTs were larger among women and individuals who were older, had highest baseline HbA1c, and used continuous glucose monitoring. ATT for HbA1c variability (-0.016 % [-0.028 to-0.0041);-0.17 mmol/mol [95 % CI-0.30 to-0.045]) corresponded to a 6.5 % decrease in the standard deviation of HbA1c. ATTs for DKA and SH corresponded to 0.52 additional and 0.11 fewer hospitalisations per 1,000 person-years, respectively.Conclusions: IPT significantly reduced HbA1c level and variability, compared with MDI. However, it also marginally increased the risk of hospitalised DKA.