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BACKGROUND: Vitamin D deficiency is a common finding in obese subjects even before any bariatric operation. However, most previous studies reporting on high rates of vitamin D deficiency in obese subjects have not systematically controlled for seasonal variations. Furthermore, the existence of seasonal variation in serum 25-hydroxyvitamin D(3) levels has not been well documented in obese subjects so far. METHODS: Serum 25-hydroxyvitamin D(3) levels were measured in 248 obese subjects (body mass index: range, 30.1-68.9 kg/m(2)). Fat mass was determined using standard bioelectrical impedance analysis. RESULTS: Serum 25-hydroxyvitamin D(3) levels decreased with the increasing body mass index and fat mass (both P < 0.001) and showed a marked variation across the seasons of the year (P < 0.001), which was not affected by the degree of obesity. According to the variation in absolute levels, the prevalence of vitamin D deficiency (<50 nmol/l) was 3.8-fold higher during winter than during summer (91.2% vs. 24.3%; P < 0.001). CONCLUSION: Data show a marked seasonal variation in absolute serum 25-hydroxyvitamin D(3) concentrations and prevalence of vitamin D deficiency in subjects with mild to extreme obesity. Considering the increasing number of studies reporting on vitamin D deficiency in obesity, the present finding points to season as a crucial factor that should not be neglected when assessing serum levels of this vitamin in obese subjects.