To evaluate the prevalence of autoimmune diseases (AD) associated with type 1 diabetes mellitus (T1DM).Analytical cross-sectional study, nested in a multicenter prospective cohort of 1121 adults with DM1 with active follow-up in endocrinology clinics. Sociodemographic and clinical variables and the presence of AD were analysed in 2010 and 2020.In this second analysis, 49,5% were male, mean age was 49.4 ± 12.8 years, median T1DM duration was 27,1 years (20,7-35,1) and mean glycated hemoglobin was 7.66 ± 1.06%. There is an absolute increase of 13% (95% CI 11-15) (p0.001) of patients with at least one AE and an absolute increase of 11.6% (95% CI 9.7-13.5) (p0.0001) of any type of autoimmune thyroid disease (ATD) after 10 years of follow-up. Likewise, the prevalence of celiac disease, autoimmune gastritis and other AD increased statistically significantly. In the multivariate logistic regression analysis, the factors that were independently associated with the presence of ATD were female gender [OR 2.9 (95% CI 2.3-3.7); p0.0001] and the presence of type 1 b diabetes (OR 0.5 [95% CI 0.3-0.9]; p = 0.041).After 10 years of follow-up, there is a substantial increase in other types of AE in patients with DM1. It seems necessary to carry out a systematic screening of these AD to optimize the follow-up of patients with 1 TDM, mainly of the ATD.