The collection of studies outlined above begins to build the case that vitamin D is likely to play a role in proper development and function of the immune system. Modern swine production is dominated by confinement operations where pigs have no direct exposure to natural sunlight. It is becoming increasingly well established that in most, if not all, weaned piglets tested for 25-OHD, levels are extremely low. One study10 reports reference values for 25-OHD of 25 to 30 ng per mL. The same study reported values of 8.42 to 13.75 ng per mL, depending on the season, for nursery-aged pigs raised indoors, and 58.54 ng per mL for 10 nursery age pigs reared outdoors in 2011. The objective of this case report is to describe findings from one farm where oral vitamin D3 supplementation was provided to piglets at 2 days of age.