Objective:To investigate the relationship between serum 25-hydroxyvitamin D[25(OH)D]levels and the plasma atherogenic index(AIP)in elderly patients with type 2 diabetes mellitus(T2DM).Methods:In our study, we recruited 848 elderly in patients with T2DM who were admitted to the Department of Geriatrics at the Second Xiangya Hospital of Central South University in Changsha, China, between January 2020 and December 2022.We measured serum levels of 25(OH)D, total cholesterol(TC), triglycerides(TG), high-density lipoprotein cholesterol(HDL-C), and low-density lipoprotein cholesterol(LDL-C), and calculated the atherogenic index of plasma(AIP)as log[TG/HDL-C].The patients were then categorized into two groups based on their AIP values: the atherogenic phenotype group(case group, AIP ≥ 0.06, 523 cases)and the non-atherogenic phenotype group(control group, AIP<0.06, 325 cases), using an AIP value of 0.06 as the cut-off point.We collected and compared clinical data from both groups, and conducted Pearson correlation analysis and multivariate Logistic regression analysis.Results:In the case group, body mass index(BMI), fasting blood glucose(FBG), glycated hemoglobin A1c(HbA1c), uric acid, TC, TG, and LDL-C levels were significantly higher than those in the control group(all P<0.01), while HDL-C and 25(OH)D levels were significantly lower(all P<0.01).AIP showed a negative correlation with 25(OH)D( r=-0.271, P<0.001).Multivariate Logistic regression analysis revealed that excluding other factors, vitamin D deficiency posed a risk for AIP( OR=2.294, 95% CI: 1.419-3.708, P=0.001). Conclusions:The level of serum 25(OH)D in elderly patients with T2DM is correlated with AIP, and vitamin D deficiency serves as an independent risk factor for AIP in this population.