Aim: Data on the prevalence of vitamin D deficiency in Lithuania are scarce. The aim was to assess the reserves of vitamin D in different age groups of out-patients, regarding the season of the year. Methods: Data on serum 25-hydroxyvitamin D (25(OH)D) levels from blood tests made in 2012-2014 were obtained from one laboratory, and a retrospective cross-sectional analysis was performed. Results: A total of 9581 subjects were included. The mean age of the participants was 33 ± 23 years. The mean

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... levels of vitamin D were higher in males than in females (p < 0.001). The highest mean 25(OH)D levels were in 0-9-year-old group, the lowest were in the 10-19-year-old group and in the group of participants that were 70 years and older (p < 0.001). The lowest vitamin D status was found in January, February, March, and April. The highest status was found in August and September. Overall, vitamin D deficiency, sufficiency, and hypervitaminosis were detected in 67%, 21%, and 12% of cases, respectively. Most cases with hypervitaminosis were in the group of children up to 2 years of age. Conclusion: Vitamin D status demonstrated clear seasonality. Significant sex-related differences of vitamin D statuses were also determined. Vitamin D deficiency was very prevalent in almost all age groups. Young children (aged up to 2 years) are of special interest for further research involving other types of 25(OH)D assays, such as those based on high-performance liquid chromatography (HPLC), since the real prevalence of "true" vitamin D hypervitaminosis in Lithuania's children is still to be determined.