We evaluated the use of dexamethasone in preterm infants to decrease morbidity associated with bronchopulmonary dysplasia in a randomized, double-blind, placebo-controlled trial. Thirty-six preterm infants (birth weight, less than or equal to 1250 g and gestational age, less than or equal to 30 weeks) who were dependent on oxygen and mechanical ventilation at two weeks of age received a 42-day course of dexamethasone (n = 13), an 18-day course of dexamethasone (n = 12), or saline placebo (n = 11). The starting dose of dexamethasone was 0.5 mg per kilogram of body weight per day, and it was progressively lowered during the period of administration. Infants in the 42-day dexamethasone group, but not those in the 18-day group, were weaned from mechanical ventilation significantly faster than control infants (medians 29, 73, and 84 days, respectively; P less than 0.05), and from supplemental oxygen (medians 65, 190, and 136 days, respectively; P less than 0.05). No clinical complications of steroid administration were noted. Follow-up of all 23 survivors at 6 and 15 months of age showed good outcome (normal neurologic examinations and Bayley Developmental Indexes greater than or equal to 84) in 7 of the 9 infants in the 42-day dexamethasone group, but in only 2 of the 9 infants in the 18-day dexamethasone group and 2 of the 5 in the placebo group (P less than 0.05). We conclude that dexamethasone therapy for 42 days improves pulmonary and neurodevelopmental outcome in very-low-birth-weight infants at high risk for bronchopulmonary dysplasia.