In this study, we aimed to reveal the proinflammatory effects of serum 25-hydroxyvitamin D3 (Vit D) deficiency and insufficiency in new-onset type 2 diabetes mellitus (T2DM) and prediabe-tes. We recruited 84 prediabetes patients, 94 new-onset T2DM patients and 113 healthy partici-pants. Levels of C-reactive protein (CRP), fibrinogen, ferritin, interleukin-1β (IL-1β), interleu-kin-6 (IL-6), interleukin-8 (IL-8), tumour necrosis factor-α (TNF-α), nuclear factor kappa-B (NF-κB) and mitogen-activated protein kinase (MAPK) were measured in the serum of the par-ticipants. The ANOVA Bonferonni and Kruskal Wallis Dunn tests were used for the comparison of the inflammation markers and vitamin D levels between the groups. Upon covariance analy-sis with age, gender and BMI; Vit D levels of T2DM group were found to be significantly lower (p<0.003). Proinflammatory markers and CRP were found to be significantly higher in predia-betic and diabetic subjects. In the prediabetes group, IL-1β, IL-6, IL-8, TNF-α and MAPK were significantly higher in those with Vit D insufficiency and deficiency groups. In T2DM group, IL-1β, IL-6, IL-8, TNF-α, NF-κB, MAPK and CRP were significantly higher in those with Vit D insufficiency and deficiency. Our study emphasizes the proinflammatory effects of Vit D on the initiation of inflammation in both prediabetes and diabetes mellitus.