OBJECTIVE: To determine the associations between age at first postnatal corticosteroids (PNS) exposure and risk for severe bronchopulmonary dysplasia (BPD) and neurodevelopmental impairment (NDI). STUDY DESIGN: Cohort study of 951 infants born <27 weeks gestational age at NICHD Neonatal Research Network sites who received PNS between 8 days of life (DOL) and 36 weeks' postmenstrual age was used to produce adjusted odds ratios (aOR). RESULTS: Compared with infants in the reference group (22-28 DOL-lowest rate), aOR for severe BPD was similar for children given PNS between DOL 8 and 49 but higher among infants treated at DOL 50-63 (aOR 1.77, 95% CI 1.03-3.06), and at DOL ≥64 (aOR 3.06, 95% CI 1.44-6.48). The aOR for NDI did not vary significantly by age of PNS exposure. CONCLUSION: For infants at high risk of BPD, initial PNS should be considered prior to 50 DOL for the lowest associated odds of severe BPD.