There are several studies in which a correlation between maternal vitamin D deficiency and serum mineral disorders in the mother and the newborn has been reported. The present randomised clinical trial was designed to investigate the effect of vitamin D administration on maternal and fetal Ca and vitamin D status. The trial was carried out on 160 pregnant women. Vitamin D-deficient (25-hydroxyvitamin D (25(OH)D) < 30 ng/ml) pregnant women were recruited at 26-28 weeks of pregnancy. In the control group, a multivitamin supplement containing 400 IU vitamin D3/d was given. Patients in the treatment group were treated with 50 000 IU vitamin D3 weekly for a total duration of 8 weeks. At delivery, maternal and fetal Ca and 25(OH)D levels in both groups were compared. In total, 81 % of pregnant women were vitamin D deficient. At the time of delivery, Ca and vitamin D levels were higher in the treatment group compared with the control group (92 (SD 3) v. 85 (SD 4) mg/l, respectively, P= 0·001 for serum Ca; 47·8 (SD 11·1) v. 15·9 (SD 6·6) ng/ml, respectively, P< 0·001 for vitamin D). At the time of delivery, 32·7 % of women in the control group had hypocalcaemia, while no hypocalcaemic case was detected in the vitamin D-treated group. Mean neonatal serum 25(OH)D was higher in the treatment group compared with the control group (27·7 (SD 5·2) v.10·9 (SD 4·4) ng/ml, respectively, P< 0·01). The neonatal Ca level in the treatment group was significantly higher than that of the control group (99 (SD 3) v. 91 (SD 3) mg/l, respectively, P< 0·001). The administration of vitamin D to pregnant women with vitamin D deficiency improves both maternal and neonatal Ca levels.