Clinical research suggests that vitamin D deficiency correlates with mental illnesses.The aim was to prove that the patients from the psychiatric health care service in Serbia had higher vitamin D deficiency than patients from general practice.The study had a cross-sectional design.The study included 47 patients aged 19 - 76 of both sexes with different mental disorders. We performed sample size calculation on available data for vitamin D deficiency in patients in health care facilities compared with the general population. The concentrations of vitamin D in serums were measured by HPLC (high performance/pressure liquid chromatography).The mean value of vitamin D (standard deviation) in the whole group of study subjects was 16.27(10.62) ng/mL; 68.1% of the patients had a deficiency of vitamin D (25(OH)D<20 ng/mL). The difference is statistically significant from expected proportion of people with vitamin D deficiency in general practice (p=0.040). Serum concentrations of 25(OH)D were significantly correlated with serum concentrations of phosphorus (ϱ=0.336, p=0.024) and sodium (ϱ=0.304, p=0.038).The patients of psychiatry health care had significantly higher frequency of vitamin D deficiency than expected. There is a significant association between serum levels of vitamin D, and phosphate and sodium.