Aim of the study: The aim is to examine the frequency of individual HLA alleles in people with T1DM and associated autoimmune diseases of the thyroid, digestive system, and skin, and to examine differences in gender and age at the time of diagnosis. Study structure: Cross-sectional study with historical data. Subjects and methods: Patients were referred to the Clinical Institute of Transfusion Medicine for HLA typing. Data were collected in Clinical hospital center Osijek (HLA typing findings, associated autoimmune diseases of the thyroid gland, digestive system, and skin; age and sex). Results: The study was conducted on 147 subjects with T1DM (51.7% women, 48,3% men). The AI disease of thyroid were most common (24,5%). Distribution of subjects by age groups differs significantly. The most common HLA alleles in men are HLA-DRB1*03, -DQA1*05, - DQB1*02, and in women HLA-DRB1*03, -DQA1*03, -DQB1*03(DQ8). Subjects with T1DM were more likely to have the DRB1*03-DQA1*05-DQB1*02 haplotype, and female subjects with T1DM were more likely to have the DRB1*08-DQA1*03-DQB1*03(DQ8) haplotype. There is no significant difference in the distribution of subjects with respect to the presence of autoimmune AI thyroid disease versus HLA-DQB1*03(DQ8); but there is significant difference in the distribution of AI of the digestive system disease relative to HLA-DRB1*1; AI skin disease versus HLA-DQB1*05. Conclusion: Individual HLA alleles have been associated with the earlier onset of T1DM and AI disease. The incidence of T1DM in the population is increasing and proper understanding of the mechanism of occurrence is important for early diagnosis of T1DM and associated AI diseases.