To evaluate the effect of maternal dietary vitamin D intake on infant vitamin D status in a country with a temperate climate, but where the commercial milk supply is not vitamin D fortified, this randomized, double-blind study was conducted on term mother-infant pairs during the winter months. Well-nourished, white nursing mothers were given a placebo, 500 IU vitamin D/day or 1,000 IU vitamin D/day; their infants were not given supplemental vitamin D. After six weeks, mothers receiving supplemental vitamin D had higher levels of 25-hydroxyvitamin D than had mothers receiving placebo. A direct relationship was observed between maternal and infant levels of 25-hydroxyvitamin D at six weeks, implying that maternal vitamin D intake directly affects the vitamin D concentration in breast milk. A control group of infants who had received 400 IU vitamin D/day had even higher concentrations of 25-hydroxyvitamin D, suggesting that infant supplementation with vitamin D is more efficacious than maternal supplementation. Despite the favorable climate in South Africa, during winter breast-fed infants have low serum vitamin D values if maternal dietary vitamin D intake is low.