The effect of supplementation with daily doses of vitamin D₂ (1000 IU or 3000 IU, 25–75 μg, 63–189 nmol) has been studied in 39 premature neonates, initial gestational age 25–32 weeks. The initial mean plasma 25‐hydroxyvitamin D was 25.8 nmol/l (10.3 ng/ml) but in 12 infants, most of whom were born in the winter months, the level was < 15 nmol/l (6 ng/ml), and in seven babies plasma 1,25‐dihydroxyvitamin D was <48 pmol/l (20 pg/ml). These findings suggest a considerable degree of maternal vitamin D‐deficiency. Maximum attained concentrations of 25‐hydroxyvitamin D on treatment were 77.3 nmol/l (30.9 ng/ ml), high dose and 86.8 nmol/l (34.7 ng/ml), low dose; the mean rate of increase was greatest during the first two weeks (2.2 nmol/l/d; 0.88 ng/ml/d) and declined over the next 4 weeks. Mean maximum concentrations of 1,25‐dihydroxyvitamin D₂ were 283 pmol/l, (121 pg/ml), high dose and 309 pmol/l (129 pg/ml), low dose. Apart from a minor contribution to the initial plasma 1,25‐dihydroxyvitamin D concentration, no effect of gestational age could be discerned in any of the measured variables. The endogenous pool of 1,25‐dihydroxyvitamin D₃ decayed with a T of 22.5 d, indicating that vitamin D supplementation of these infants was necessary to avoid vitamin D‐deficiency.