Objective: To test the hypothesis that antenatal dexamethasone treatment to promote fetal lung maturation results in decreased birth weight corrected for gestational age. Methods: The birth weights of all dexamethasone-treated, singleton, live-born infants delivered at our hospital were compared with our overall obstetric population; a group of untreated infants frequency matched approximately 3:1 according to maternal race, infant sex, and gestational age at delivery; and an historical cohort of infants with an indication for dexamethasone but delivered in the 12 months before the introduction of corticosteroid therapy at our hospital. Results: Dexamethasone-treated infants (n = 961), when compared with either the overall population (n = 122,629) or matched controls (n = 2808), had significantly lower birth weights after adjustment for week of gestation (P < .001). Compared with the historical cohort of infants, the average birth weight of dexamethasone-treated infants was smaller by 12 g at 24–26 weeks, 63 g at 27–29 weeks, 161 g at 30–32 weeks, and 80 g at 33–34 weeks’ gestation. Conclusion: Antenatal dexamethasone administered to promote fetal maturation is associated with diminished birth weight.