Background: Polish data on vitamin D deficiency in the population are incomplete. Vitamin D deficiency seems to be common, but there is a lack of studies concerning the concentration of 25(OH)D in people with high UV exposure. The aim of this study was to evaluate the plasma concentration of 25(OH)D in people with and without metabolic syndrome (MS), working in agriculture, the prevalence of its deficiency in these workers, and the correlation between the plasma concentration of 25(OH)D and traditional biomarkers of cardiovascular diseases. Material and Methods: The study included 332 people working in agriculture in the Łódź voivodeship, including 231 people with MS and 101 healthy ones. The plasma concentration of 25(OH)D was assessed using the chemiluminescent immunoassay technology. The vitamin D intake was assessed using a 24-h recall questionnaire using Diet 5.0 software. Results: The mean plasma concentration of 25(OH)D was 13.64±8.01 ng/ml in MS workers, and it was significantly lower than in the healthy ones (26.61±10.12 ng/ml, p < 0.00001); the highest concentration of 25(OH)D was noted in summer months. Deficient plasma concentrations of 25(OH)D were found significantly more often in MS workers than in the controls (81.82% and 20.79%, respectively, p < 0.00001). No correlation was found between the plasma vitamin D concentration and its dietary intake. The plasma concentration of 25(OH)D correlated with age of the examined workers (r = –0.28, p = 0.023), high density lipoprotein concentration (r = 0.19, p = 0.036) and glucose concentration (Rho = –0.24, p = 0.02). A multivariate analysis of variance demonstrated that the body mass index affected significantly the mean value of the 25(OH)D concentration in MS workers. Conclusions: The concentration of vitamin D in the plasma of workers with MS was significantly lower than in the healthy controls despite the same high UV exposure; these workers also manifested significantly higher 25(OH)D deficiency than the control subjects. This study indicates the need for further research on the concentration of 25(OH)D in people with metabolic disorders regardless of UV exposure and vitamin D intake with a diet. Med Pr. 2021;72(1):9–18