Seasonal changes were observed in the vitamin D status of mothers and their newly born infants in sunny Israel. Maternal and newborn serum levels of 25-hydroxyvitamin D (25-OHD) were lower (less than 0.01) in March-April (n= 45 pairs), than in September-October (n= 33 pairs). Parallel seasonal changes were also found in serum 24, 25-dihydroxyvitamin D [24, 25-(OH) 2D] concentrations. In the spring, 20% of the mothers and 40% of their infants had vitamin D deficiency or borderline serum 25-OHD levels. In the autumn, in contrast, none of the mothers and only one newborn were vitamin D-deficient, and one mother and two newborns had borderline serum 25-OHD levels. The results demonstrate that even in Mediterranean climates there are seasonal changes in maternal vitamin D status, which have a significant effect on the serum levels of vitamin D metabolites in their newborn infants. This raises the question as to whether vitamin D supplements should be given to pregnant women in Israel, at least during the winter.