Vitamin D insufficiency and deficiency are highly prevalent worldwide, and pregnant women and infants are most at risk. There is consistent experimental data that vitamin D affects fetal lung and immune development, thus, we hypothesized that vitamin D supplementation in pregnancy would help to prevent the development of asthma and improve lung function in children. VDAART showed that there was an early effect of prenatal vitamin D supplementation on decreasing asthma and recurrent wheezing through age 3 years, but that this effect waned as the children grew older. We have also reported that prenatal vitamin D supplementation had effects on lung function growth, and in particular, that it counteracted the effects of cigarette smoke exposure in pregnancy. These results will be discussed, and the limitations of the trial and other nutrient trials will be highlighted to improve study designs for future trials.