Tehran University of Medical Sciences Journal of Craniomaxillofacial Research 2345-5489 12 1 2025 08 25 30 36 Saeed Soleimani Department of Oral and Maxillofacial Medicine, Dental Research Center, School of Dentistry, Hamadan University of Medical Sciences, Hamadan, Iran. Mina Jazaeri Department of Oral and Maxillofacial Medicine, Dental Research Center, School of Dentistry, Hamadan University of Medical Sciences, Hamadan, Iran. Shiva Borzoui Department of Endocrinology, School of Medicine, Hamadan University of Medical Sciences, Hamadan, Iran. Salman Khazaei Department of Epidemiology, School of Health, Hamadan University of Medical Sciences, Hamadan, Iran. Zahra Pourgholi Takrami Department of Oral and Maxillofacial Medicine, Dental Research Center, School of Dentistry, Hamadan University of Medical Sciences, Hamadan, Iran. 2025 04 01 2025 07 12 Introduction: The leading cause of tooth decay is Streptococcus mutans bacteria. Diabetes is also a condition that can impact the mouth’s microbiology and the composition of saliva. The studies on the relationship between these two variables are limited; therefore, the present study was conducted to investigate the correlation between glycosylated hemoglobin A1c and the count of mutans streptococci in the mouths of patients with type 2 diabetes.  Materials and Methods: The present study was a cross-sectional analytical study conducted in the specialized diabetes clinic of Hamadan University of Medical Sciences, Iran. Sixty people with type 2 diabetes, non-smokers, who were willing to participate in the study to check the count of Streptococcus mutants, had samples collected from three different locations. Hemoglobin A1c and other relevant information were also extracted from the patients’ files, and the data were analyzed using SPSS 24.  Results: There were significant differences in Streptococcus mutans counts among the dorsal surface of the tongue, the gingival groove and the mandibular buccal vestibule (p=0.001), the buccal vestibule and the dorsal surface of the tongue (r=0.337, p=0.008) and the buccal vestibule of the lower jaw and the gingival groove (r=0.361, p=0.004). No significant relationship was observed between the number of Streptococcus mutans bacteria on the dorsal surface of the tongue and the gingival groove (r=-0.197, p=0.137). Conclusion: According to the findings of this study, although HbA1c is associated with poor control of diabetes mellitus, there was no significant correlation between Streptococcus mutans counts and HbA1c levels. Furthermore, it can be concluded that Streptococcus mutans counts are closely related to specific areas within the oral cavity.  Keywords: Diabetes mellitus; Type 2; Glycated hemoglobin; Hb A1c; Streptococcus mutans. https://jcr.tums.ac.ir/index.php/jcr/article/view/535 https://jcr.tums.ac.ir/index.php/jcr/article/download/535/438