Objectives. To evaluate i) the correlation between vitamin D (vit. D) serum concentra-tions and metabolic syndrome (MetS); ii) the efficacy of 6 months supplementation therapy with vit. D. Method. 200 patients were enrolled. Blood analyses and anthropometric measurements were carried out. Patients with hypovitaminosis D received an oral supplement therapy. Results. 81% of the sample shows vit. D levels < 30 ng/mL. Rate of MetS was significant-ly higher in vit. D deficiency group than in vit D insufficiency (p = 0.009) and sufficiency (p = 0.002) groups. Vit. D shows a significant negative correlation with both waist circum-ference (WC) (ρ - 0.202 p = 0.004) and glycaemia values (FBG) (ρ -0.185 p = 0.009). After the supplementation therapy in a group of 60 subjects a significant increase in vit. D levels (p = 0.001) and a significant reduction in WC values (p = 0.001) were observed. Conclusions. MetS, WC and FBG appeared to be associated vit. D status and it is well-known that central obesity, with the inflammatory alterations thereto correlated that determine insulin resistance, can be considered the “primum movens” for the develop-ment of MetS.