The aim of this research was to determine the presence of seasonal variations of the concentration of vitamin D in the population of patients in Split – Dalmatia County based on the data from Clinical Hospital Centre Split. The research included patients whose concentration of vitamin D was determined in the period from January 1st 2017 to December 31st 2018. Patients were excluded from this research based on these criteria: children under the age of 18, hospitalized patients and patients from the oncology ward. The concentration of vitamin D was determined by competitive immunoassay (Elecsys® Vitamin D total test). This test quantitatively determines the total concentration 25(OH) of vitamin D in the human serum and plasma. The data was analysed by MedCalc Software. The results show similar trend in the variation of the concentration of vitamin D across quarters in both years included in the research. A slight drop in values of median in the second quarter could be seen during 2017, while in 2018 there was a continuous rise until the last quarter. The highest values of the median were recorded in the period from July to September, in the summer period. In the autumn, the median values of vitamin D concentration decrease. By observing the results, we have determined that there is a seasonal variation of the concentration of vitamin D in the study population. The highest concentration of vitamin D is present in the summer months because of the abundant exposure to the sunlight and stronger UV radiation, while during the autumn the concentration gradually decreases. During the winter period, from January to March, the concentration of vitamin D is the lowest. In the spring time, when it is warmer, people spend more time outside and according to that, the concentration of vitamin D elevates. Research has shown that, despite the great number of sunshine hours in Dalmatia, there are significant seasonal variations in concentration of vitamin D. The results obtained by this research are consistent with the most studies that analyze the seasonal change in the concentration of vitamin D.