Objective: The aim of this study was to investigate the role of vitamin D replacement therapy on advanced glycation end-products in prediabetes and type 2 diabetes mellitus patients with vitamin D deficiency. Methods: One hundred twenty subjects with serum 25-hydroxyl vitamin D levels less than 20 ng/mL were included in the study. Forty type 2 diabetes mellitus patients (type 2 diabetes mellitus group), 40 prediabetes patients (prediabetes group), and 40 non-diabetes controls (non-diabetes group) were given oral vitamin D3 50 000 units/week for 8 weeks as loading and followed by 1500 U/day as a maintenance dose. We measured serum 25-hydroxyl vitamin D, glycated hemoglobin, carboxymethyl lysine levels, and skin autofluorescence before and on the fourth month of the therapy. Results: Basal serum carboxymethyl lysine and skin autofluorescence measurement in type 2 diabe- tes mellitus and prediabetes groups were significantly higher than the non-diabetes group. While no difference was found between glycated hemoglobin and skin autofluorescence, serum carboxy- methyl lysine levels were significantly elevated for each group following vitamin D replacement. Conclusions: Vitamin D loading did not affect skin advanced glycation end-product levels and gly- cated hemoglobin but was associated with increased serum carboxymethyl lysine levels in all groups.