Vitamin D deficiency is prevalent in extremely preterm infants and may contribute to adverse outcomes. We conducted a single-center, blinded comparative effectiveness trial of inborn infants <28 weeks’ gestation or <1000 g birth weight randomized within 96 postnatal hours. Infants were stratified by birth weight (<750 vs ≥750 g) and randomized 1:1 to receive either routine dose (400 IU/day with established feedings–the most common regimen in the US–and saline placebo) or earlier and higher dose (800 IU/day starting with any feedings) for the first 28 postnatal days. The primary outcome was serum 25(OH)D at one postnatal month assessed by liquid chromatography–tandem mass spectrometry. Secondary outcomes included death before discharge, survival with grade of bronchopulmonary dysplasia (ordinal outcome), and other prespecified in-hospital outcomes. Bayesian analyses were performed using a neutral prior. Between September 2022 and June 2025, 180 infants were randomized (90 per group, mean±SD gestational age 25± wk; birth weight 773± 210 g; baseline 25(OH)D 18±7 ng/ml). At one postnatal month, the earlier and higher-dose group achieved higher mean serum 25(OH)D levels (67 vs 32 ng/mL, probability of benefit: >99%) and had lower vitamin D insufficiency (defined as 25(OH)D <30 ng/ml; 2% vs 59%, probability of benefit: >99%). Earlier and higher-dose supplementation was associated with an OR of 1.19 (95% credible interval, 0.72-1.99) for improved survival with less severe BPD, with a probability of benefit of 75 %. Earlier and higher-dose supplementation compared to the routine dose showed favorable trends for multiple outcomes: mortality (6% vs. 10%, probability of benefit: 81%), grade 2-3 BPD (42% vs 48%, probability of benefit: 77%), fractures (4% vs 11%, probability of benefit: 91%), spontaneous intestinal perforation (6% vs 14%, probability of benefit: 95%), and bacterial sepsis (18% vs 27%, probability of benefit: 90%). No clinical vitamin D toxicity was identified. In conclusion, extremely preterm infants randomized to the earlier and higher-dose vitamin D supplementation had higher serum 25(OH)D levels at one postnatal month. The earlier and higher-dose regimen had a high probability of improving several important clinical outcomes, including mortality, bronchopulmonary dysplasia, spontaneous intestinal perforation, fractures, and bacterial sepsis, compared to routine vitamin D dosing. These findings warrant confirmation in a larger multicenter randomized controlled trial.