Objectives: Type 1 diabetes mellitus is a chronic disorder associated with development of autoimmunity. In this work, we studied the relationship between severity of acidosis at diagnosis and future risk for autoimmunity development in children with type 1 diabetes. Methods: We investigated the presence of associated autoimmunity in 144 children with type 1 diabetes (mean +/- standard deviation: age, 12.44 +/- 4.76 years; diabetes duration, 4.41 +/- 3.70 years). We identified the presence of thyroid disease, celiac disease, autoimmune gastritis and adrenal autoimmunity, and retrospectively reviewed the files for presence of diabetic ketoacidosis at diagnosis. Results: Autoimmunity prevalence was 16.7% for thyroid autoimmunity, 9.5% for celiac disease, 5% for gastric autoimmunity and 8.0% for multiple autoimmunities. There were strong associations between severe acidosis at diabetes diagnosis (pH<7.10) and development of thyroid autoimmunity (odds ratio [OR], 5.34; 95% confidence interval [CI], 1.90-15.1; p<0.001), celiac disease (OR, 5.83; 95% CI, 1.19-28.6; p=0.013), gastric autoimmunity (OR, 13.1; 95% CI, 1.22-140; p=0.006) and multiple autoimmunity (OR, 26.7; 95% CI, 2.36-301; p<0.01). The associations persisted after adjustment for sex, age at diabetes diagnosis, age at assessment, time since diabetes diagnosis and antiglutamic acid decarboxylase autoantibody status. Conclusions: The severity of acidosis at diagnosis is strongly associated with the development of associated autoimmune diseases in children with type 1 diabetes and could act as a predictive factor for multiple autoimmunity development. This association can be either due to effect of acidosis on immune system or to the presence of a more aggressive diabetes endotype. (C) 2020 Canadian Diabetes Association.