Continuing differences of opinion among obstetricians and neonatologists about the place of corticosteroid administration before preterm delivery have prompted us to carry out a systematic review of the relevant controlled trials, using methods designed to minimize systematic and random error. Data from 12 controlled trials, involving over 3000 participants, show that corticosteroids reduce the occurrence of respiratory distress syndrome overall and in all the subgroups of trial participants that we examined. This reduction in respiratory morbidity was associated with reductions in the risk of intraventricular haemorrhage, necrotizing enterocolitis and neonatal death. There is no strong evidence suggesting adverse effects of corticosteroids. The risks of fetal and neonatal infection may be raised if they are administered after prolonged rupture of the membranes, but this possibility is not substantiated by the results of the available trials. The available data on long-term follow-up suggest that the short-term beneficial effects of corticosteroids may be reflected in reduced neurological morbidity in the longer term.