The serum 25-hydroxyvitamin D [25(OH)D] concentration was measured in a representative sample of the general adult population in Switzerland (n = 3276). The median concentration was 46 nmol/L and no significant difference was found between men and women. Between the ages of 25 and 74 y, 25(OH)D was not significantly correlated with age (r2 = 0.01). Six percent of the population was vitamin D deficient [ie, 25(OH)D less than or equal to 20 nmol/L] and between 34% and 95% had a relatively low concentration of vitamin D (ie, less than 38 or less than 95 nmol/L, respectively). Among the determinants of low 25(OH)D were indicators of little sunshine exposure, such as the winter season [odds ratio (OR) 2.4, 1.5-3.7 (95% confidence interval)] and less than 30 min of time spent outdoors daily by individuals greater than 65 y of age (OR 5.6, 1.5-21.2), as well as indicators of low nutritional intake of vitamin D, such as the absence of regular intake of butter or margarine (OR 2.0, 1.3-3.1) and the consumption of few dairy products (OR 1.7, 1.2-2.4).