Life and work of humans in high latitudes are often associated with adverse conditions such as very cold climate, changed circadian cycle, and altered exposure to ultraviolet (UV) light. In addition, extended human stays in Antarctic research stations may be associated with psychosocial isolation, sensory deprivation, and exhaustion. Polar regions receive less‐intensive solar radiation because the sunlight hits the Earth at an oblique angle. In addition, the Antarctic climate is dominated by seasonal changes. Months of complete darkness during the Antarctic winter alternate with months of 24‐hour bright daylight in the Antarctic summer. This has particular consequences on vitamin D homeostasis for humans residing there. The collective term "vitamin D" (calciferol) combines vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). The formation of vitamin D in the human skin makes up to 95% of the vitamin D requirement, indicating the importance of adequate UV light for vitamin D formation. A photochemical conversion of the provitamin D3 (7‐dehydrocholesterol 7‐DHC) by UV light of wavelengths 290–315 nm causes the formation of pre‐vitamin D3, which is converted to vitamin D3 through thermal isomerisation. In the liver and kidneys the final activation steps to 1,25‐ dihydroxyvitamin D (calcitriol) are catalysed. 1,25‐dihydroxyvitamin D has calcaemic and non‐calcaemic effects. The former are to maintain the calcium and phosphate homeostasis through regulation of intestinal and renal calcium absorption, bone tissue calcification, and inhibition of parathyroid hormone. The latter serve to regulate cell growth and differentiation, to regulate immune function, to control the renin‐angiotensin system, and to control muscular function, brain development, and mood. Positive effects of vitamin D have been shown on the nervous system, inhibition of diseases such as the metabolic syndrome, susceptibility to infection, and several types of cancer. Genetically low vitamin D serum concentrations seem to be associated with increased all‐cause mortality. 25‐OH‐vitamin D serum concentration has been accepted and used as an accurate measure of a human’s vitamin D content, which considers both intake from diet and skin production. Serum concentrations of 25‐OH‐vitamin D of at least 75 nmol/l have been shown to effectively prevent fractures and are seen by some authors as the lower limit to maintain health. Lower, more conservative thresholds of at least 50 nmol/l, have been introduced as being sufficient. Around one billion people worldwide are estimated to be vitamin D deficient. Wearing cold‐protective clothing, shielding the skin for cultural or religious reasons, residence at high latitudes, and having dark skin pigmentation are known to increase the risk for vitamin D deficiency. Especially at higher latitudes, during local wintertime, vitamin D production requires longer exposure time to UV light or it ceases completely. The German Antarctic research stations Georg‐von‐Neumayer II (Neumayer II) and III (Neumayer III), located at 70° 40’ S, 08° 16’ W, served as the site of the presented study. The aim of the study was to assess changes in 25‐OH‐vitamin D serum concentration in winter‐over personnel of the stations for 13 months during a total of six campaigns, from 2007 to 2012. We hypothesised that vitamin D would be significantly decreased during the Antarctic winter. Furthermore, we assessed whether these changes were affected by age, gender, baseline fat mass, baseline 25‐OH‐vitamin D serum concentration, and the type of inhabited station (station II was located below ground; station III was located above ground).