Vitamins are organic substances that the body needs in small quantities for it to function properly. Vitamin D plays a major role in maintaining the immune system and treating a large number of clinical conditions. Aim of the research: to show the frequency of requirements for the determination of vitamin D in 2019 and 2020, the average value of vitamin D with regard to the seasons and the prevalence of medical diagnoses requiring the determination of vitamin D in the blood for PHC patients. Data were collected in the Department of Medical Biochemical Activity of the General Hospital Pula. Respondents are PHC patients during 2019 and the first eight months of 2020. Vitamin D concentration was determined in serum samples, by measurement by electrochemiluminescence method. When processing the results, the MedCalc R statistical program and the software version of Excel 2016 were used. The obtained results are presented by the mean value, the coefficient of variation and the percentage. The largest number of vitamin D determinations are in patients in primary health care. The frequency of vitamin D determination is significantly higher in the female population compared to the male population. The most common diagnosis that requires the determination of vitamin D in outpatients is other hypothyroidism, and in inpatients at the very top is a cerebral infarction due to cerebral artery thrombosis. The mean value of vitamin D determination in men and women, regardless of where they come from, is 67.8 nmol / L, which indicates the existence of generalized hypovitaminosis D.