Vitamin D nutrition in pregnancy was investigated in 115 Asian women living in London, and in 50 of their newborn infants. Mean serum 25-hydroxy cholecalciferal (25-OHD) concentration at the beginning of the last trimester was 20.2 nmol/l, and fell to 16.0 nmol/1 post partum. Thirty-six per cent of the women post partum and 32 per cent of the infants had undetectable 25-OHD concentrations (less than 3 nmol/l). Such low values are commonly associated with osteomalacia and rickets. The bone isoenzyme of alkaline phosphatase was elevated in 20 per cent of the women post partum, and in 50 per cent of the infants, indicating the presence of sub-clinical bone disease. Five infants had symptomatic hypocalcaemia. Vitamin D deficiency was most likely to occur in Pakistanis, Hindu Indians and East African Asians, and in vegetarians. All British Asians should receive supplementary Vitamin D during pregnancy.