Two groups of white, primiparous women and their babies were studied: one group in April 1979 and the other in September 1979. They were selected to be as near normal as possible. In each case maternal and cord blood samples were taken at delivery and analysed for serum 25-hydroxycholecalciferol (25-OHD), calcium, magnesium, phosphate, alkaline phosphatase, total protein, and albumin. Follow-up was by questionnaire at 6 weeks. The study showed a highly significant increase in maternal and cord serum 25-OHD levels in September. The few mothers who had taken vitamin D supplements had significantly higher serum 25-OHD values. Some of the unsupplemented women studied in April had low serum 25-OHD levels suggesting that oral vitamin D supplements should be given to pregnant white women in Britain, at least during the winter.