The purpose of this randomized study was to evaluate whether the combined use of corticosteroids and thyrotropin- releasing hormone would enhance fetal lung maturation to a greater degree than would corticosteroids alone. The study was restricted to patients under 34 weeks' gestation with a lecithin/sphingomyelin (L/S) ratio less than 2.0. The patients were randomized into a study group receiving intravenous thyrotropin-releasing hormone along with intramuscular corticosteroids over 48 hours and a control group receiving only corticosteroids. Patients undelivered 1 week after the onset of therapy underwent a repeat amniocentesis to document changes in the L/S ratio. In those patients delivering within 1 week of therapy, the neonatal clinical course was established by two investigators blinded to the antenatal therapy. Compared with the control group, the group receiving antenatal corticosteroids plus thyrotropin- releasing hormone showed a greater post-therapy increase in L/S ratio, fewer respirator days, and a lower incidence of bronchopulmonary dysplasia. The results of this study suggest that the combined use of corticosteroids and thyrotropin-releasing hormone results in enhanced fetal pulmonic maturation superior to that achieved with corticosteroids alone