OBJECTIVETo assess the association between exanthematous diseases, and an earlier age at Type 1 diabetes mellitus diagnosis (T1DM) in a cohort of Brazilian patients.METHODSThis was a retrospective cohort study including 812 patients diagnosed with T1DM in Bauru, São Paulo, Brazil, between 1981 and 2023. Data regarding sociodemographic parameters such as age, sex, ethnicity, socioeconomic status, as well as the occurrence of a previous exanthematous diseases, such as chickenpox, measles, rubella, mumps and scarlet fever were collected. An adapted survival analysis was performed to evaluate the impact of each variable on the age of T1DM diagnosis.RESULTSOverall, 596 patients were evaluated. Their average age at T1DM diagnosis was 12 ± 7.69 years. It was found that presenting rubella, measles, and mumps, as well as belonging to non-high socioeconomic class, were associated with 35%, 40%, 39%, and 34% lower age at T1DM diagnosis, respectively.CONCLUSIONSThis study has found that rubella, measles, mumps, and belonging to non-high socioeconomic classes were significantly associated with earlier age at T1DM diagnosis in a cohort of Brazilian patients with T1DM. Future studies with other populations are warranted to confirm our findings.