To investigate the relation between thyroid function and respiratory-distress syndrome we determined thyroid hormone in cord serum from 39 term and 120 premature newborns. In groups matched for gestational age (33 to 37 weeks) and body length, after exclusion of newborns with birth asphyxia, maternal diabetes and delivery by cesarean section, the newborns with respiratory distress syndrome had significantly lower cord tri-iodothyronine index, higher ratio of thyroxine to tri-iodothyronine and higher thyrotropin concentration than those without syndrome (P less than 0.05 or less). Conversely, the frequency and severity of respiratory-distress syndrome were higher in newborns who had low cord tri-iodothyronine and thyroxine concentration, low free tri-iodothyronine index and high thyrotropin levels. These observations show an association between low thyroid activity at birth and respiratory-distress syndrome and are in accord with animal studies showing accelerated lung maturation due to thyroxine administration.