Aims/hypothesis We investigated the long-term impact of diabetic ketoacidosis (DKA) at onset on metabolic regulation and residual beta cell function in a Danish population with type 1 diabetes. Methods The study is based on data from DanDiabKids, a Danish national diabetes register for children. The register provides clinical and biochemical data on patients with type 1 diabetes diagnosed in 1996-2009 and then followed-up until 1 January 2012. Repeated-measurement models were used as statistical methods.Results The study population comprised 2,964 children <18 years. The prevalence of DKA at diagnosis was 17.9%. Of the total subjects, 8.3% had mild, 7.9% had moderate and 1.7% had severe DKA. DKA (moderate and severe) was associated with increased HbA 1c (%) levels (0.24; 95% CI 0.11, 0.36; p=0.0003) and increased insulin dose-adjusted HbA 1c (IDAA 1c , 0.51; 95% CI 0.31, 0.70; p<0.0001) during followup, after adjustment for covariates. Children without a family history of diabetes were more likely to present with DKA (19.2% vs 8.8%, p<0.0001); however, these children had a lower HbA 1c (%) level over time (−0.35; 95% CI −0.46, −0.24; p<0.0001). Continuous subcutaneous insulin infusion (CSII) was associated with a long-term reduction in HbA 1c , changing the effect of DKA, after adjustment for covariates (p<0.0001). Conclusions/interpretation DKA at diagnosis was associated with poor long-term metabolic regulation and residual beta cell function as assessed by HbA 1c and IDAA 1c, respectively; however, CSII treatment was associated with improvement in glycaemic regulation and residual beta cell function, changing the effect of DKA at onset in our population.