Thyroxine (T4), triiodothyronine (T3), and thyrotropin (TSH) concentrations were measured on cord sera of 21 preterm infants with idiopathic respiratory distress syndrome (IRDS) and were compared to the values obtained from 15 healthy preterm infants. When log T3, log T4, and log TSH were considered as the dependent variables in the multivariate test Hotelling-Lawley-Trace, there was an overall difference between the two groups: F = 3.94, p = 0.03. When log T3 and log T4 were considered separately in an analysis of covariance, there was a significant difference between the two groups for log T3 after adjusting for birth weight and gestational age (F = 7.98, p = 0.008). However, for log T4 and TSH, there was no difference between the IRDS and control infants. These findings exclude the possibility of antenatal thyroid dysfunction in babies with IRDS. An explanation for reduced cord blood T3 concentration in infants with IRDS is lacking at present. Extrathyroidal factors that predispose to IRDS may also affect peripheral T3 generation. Alternatively, one might postulate that there is relative immaturity of both peripheral T3 generating pathway and lung development in infants who develop IRDS.