Objective: To determine whether regulating vitamin C(ascorbic acid: AA) intak e to achieve higher or lower plasma concentrations was associated with improved clinical outcome. Design: A double blind, randomised controlled trial. Setting: Neonatal intensive care unit at Christchurch Women’s Hospital. Patients: Infan ts with birth weight < 1500 g or gestation < 32 weeks, admitted to the unit wi thin 48 hours of birth. Intervention: Infants were randomised to one of three pr otocols with regard to AA supplementation for the first 28 days of life: group L L received low supplementation throughout; group LH received low until day 10 an d then high: group HH received high throughout. Main outcome measures: Primary o utcome measures were oxygen requirement at 28 days and 30 weeks postmenstrual ag e, total days supplemental oxygen, and retinopathy of prematurity. AA concentrat ions were measured at study entry (day 2), and days 10, 21, and 28. Reults: A to tal of 119 infants were enrolled over 24 months (mean gestation 28.4 weeks; birt h weight 1161 g). Six infants died, and these had significantly higher AA concen trations before randomisation than surviving infants (116 μ mol/l (95% confid ence interval 90 to 142) v 51 μ mol/l (45 to 58), p < 0.0001). There were no si gnificant differences in primary outcomes between the groups. However, the propo rtion of surviving infants with an oxygen requirement at 36 weeks postmenstrual age in group HH(19% )- was half that in group LL (41% ) (p = 0.06). Conclusio ns: In a randomised controlled trial, no significant benefits or harmful effects were associated with treatment allocation to higher or lower AA supplementation throughout the first 28 days of life. Objective: To determine whether or not vitamin C (ascorbic acid: AA) intak e to achieve higher or lower plasma concentrations was associated with improved clinical outcome. Design: A double blind, randomized controlled trial. Infants were randomized to one of three protocols with regard to AA supplementation for the first 28 days of. group LH received low until day 10 an d then high: group HH received high throughout. Main outcome measures: Primary o utcome measures were oxygen requirement at 28 days and 30 weeks postmenstrual ag e, total days supplemental oxygen, and retinopathy of prematurity. AA concentrates were measured at study entry (day 2), and days 10, 21, and 28. Reults: A to tal of 119 infants were enrolled over Six infants died, and these had significantly more AA concen trations before randomisation than surviving infants (116 μ mol / l (95% confid ence interval 90 to 142) v 51 (mean gestation 28.4 weeks; birt h weight 1161 g) There were no si gnificant differences in primary outcomes between the groups. However, the propo rtion of surviving infants with an oxygen requirement at 36 weeks postmenstrual age in group HH (19 %) - was half that in group LL (41%) (p = 0.06). Conclusio ns: In a randomized controlled trial, no significant benefits or harmful effects were associated with treatment allocation to higher or lower AA supplementation throughout the first 28 days of life.