Background: Ascorbylperoxide (AscOOH) is a hydrogen peroxide–dependent by-product of ascorbic acid that contaminates parenteral nutrition. In a guinea pig model, it caused oxidized redox potential, increased apoptosis, and decreased alveolarization. AscOOH detoxification is carried out by glutathione peroxidase (GPX). We hypothesize that extremely preterm infants have limited capacity for AscOOH detoxification. Our objective was to determine if there is an association between an early level of urinary AscOOH and later development of bronchopulmonary dysplasia (BPD) or death. Materials and Methods: This prospective cohort study included 51 infants at <29 weeks of gestation. Baseline clinical characteristics and clinical outcomes data were collected. Urine samples were collected on days 3, 5, and 7 of life for urinary AscOOH. Blood samples on day 7 were collected for total plasma glutathione, GPX, and glutathione reductase. χ2, Student’s t test, Spearman correlation (r), linear regression (adjusted r2), and repeated-measure analysis of variance were used as appropriate. P < .05 was considered significant. Results: Urinary AscOOH increased over time (P = .001) and was higher in infants who later developed BPD or died (P = .037). Compared with adults and full-term infants, total plasma glutathione concentration was low (median, 1.02 µmol/L; 25th–75th percentiles, 0.49–1.76 µmol/L), whereas GPX and glutathione reductase activities were sufficient (3.98 ± 1.25 and 0.36 ± 0.01 nmol/min/mg of protein, respectively). Conclusion: Extremely preterm infants have low glutathione levels, which limit their capacity to detoxify AscOOH. Higher first-week urinary AscOOH levels are associated with an increased incidence of BPD or death.