The objectives of this investigation were to evaluate the serum 25-hydroxyvitamin D [25(OH)D] concentrations in pigs of different age groups, to determine if 25(OH)D concentrations varied with season, and to assess the quality of vitamin D supplements used in swine diets from multiple commercial suppliers. Serum samples (n = 1200) submitted to a diagnostic laboratory for routine surveillance were assayed for serum 25(OH)D concentrations. Vitamin D premix samples were obtained from suppliers and analyzed at two laboratories over a 9-month period. In all age categories, 25(OH)D concentrations in numerous serum samples were lower than reference values. In the nursery, finisher, and boar age categories, there was a difference between the months of January and June (P < .05), with June samples containing higher quantities of circulating 25(OH)D. Serum samples from outdoor herds had higher 25(OH)D concentrations than samples from confined pigs (P < .01). Among the supplement samples evaluated, no individual supplement had a concentration of 25(OH)D significantly lower than 500,000 IU per g. These results revealed that commercial swine may be deficient in serum vitamin D at varying times of the year, and feed-supplement concentrations may vary.