BACKGROUND: Despite substantial evidence that vitamin D deficiency is highly prevalent among infants born extremely preterm (≤28 weeks' of gestation), several consensus statements do not recommend vitamin D doses >400 IU/day for these infants. Safety remains a concern. OBJECTIVE: The study aim was to determine safety and efficacy profiles of enteral vitamin D in Black and White infants randomized to three different vitamin D doses soon after birth. DESIGN: Ancillary study of a masked randomized clinical trial. PARTICIPANTS/SETTING: Seventy-three infants born extremely preterm between 2012 and 2015 at a southern US academic neonatal unit (33' latitude) who had >90% compliance with the assigned intervention were included. INTERVENTION: Infants were randomized to receive placebo (placebo group), 200 IU/day vitamin D (200 IU group), or 800 IU/day vitamin D (800 IU group) during the first 28 days after birth. MAIN OUTCOME MEASURES: Safety outcomes included serum 25-hydroxy vitamin D (25[OH]D) and calcium concentrations. Efficacy outcomes included the predictive risk of bronchopulmonary dysplasia. STATISTICAL ANALYSIS: Per-protocol analysis using unadjusted, repeated-measures mixed models. RESULTS: Mean birth weight was 815 ± 199 g. Half were male and 56% were Black. Of 58 infants with 25(OH)D measurements at birth, 40 (69%) had vitamin D deficiency (<20 ng/mL). The mean difference in 25(OH)D in nanograms per milliliter between Postnatal Day 28 and Postnatal Day 1 was +9 in the placebo group, +23 in the 200 IU group, and +62 in the 800 IU group (P < 0.0001). The increase observed in 25(OH)D was more significant among Black infants. The predictive risk of severe bronchopulmonary dysplasia in the 200 IU and 800 IU groups was lower, but this difference did not reach statistical significance. No vitamin D or calcium toxicity was observed. CONCLUSIONS: A vitamin D dose of 800 IU/day safely corrected vitamin D deficiency by Postnatal Day 14.