Objective To investigate the direct effects of corticosteroids on human umbilical artery resistance, in vitro.Design Prospective laboratory study.Setting University teaching hospital.Samples and methods Umbilical artery samples were obtained following normal, term deliveries (n = 50) and dissected rings were suspended for isometric recording under physiological conditions. The effects of hydrocortisone (10(-9)10(-4) M), dexamethasone (10(-9)-10(-4) M) and betamethasone (10(-9)-10(-4) M) on umbilical artery resistance were measured in vitro.Main outcome measures Changes in umbilical artery resistance, in vitro.Results Hydrocortisone (n = 12) exerted a vasodilatory effect on human umbilical artery at all concentrations studied compared with vehicle control experiments (n = 12) (P < 0.0001). The mean net relaxant effect of hydrocortisone ranged from 11.77% (10(-9) M) to 57.01% (10(-4)). Both exogenous compounds, dexamethasone (n = 12) and betamethasone (n = 12), similarly exerted a significant relaxant effect on human umbilical artery tone (P < 0.05-0.01), compared with vehicle control experiments (n = 12). The mean net relaxant effect of dexamethasone ranged from 14.43% (10(-9) M) to 38.12% (10(-4)) and that of betamethasone ranged from 6.02% (10(-9) M) to 42.30% (10(-4)), in a cumulatively increasing fashion. There was a non-significant trend towards a greater vasodilatory effect of dexamethasone than betamethasone at lower bath concentrations studied.Conclusion Corticosteroids exert a direct and potent vasodilatory effect on human umbilical artery resistance in vitro, thus providing an explanation for the previously unexplained vascular effects associated with antenatal administration of corticosteroids.