In 51 sick newborns the influence of two different nonionic, iodine-containing contrast agents, Amipaque (group 1) and Omnipaque (group 2) and of long-term treatment with polyvinylpyrrolidone-iodine (PVP-I) (group 3) on thyroid function was studied. In the dose given, freshly dissolved Amipaque releases roughly 100 micrograms 'free' iodide/kg body weight; this release may be even higher in the solubilized agent Omnipaque because of increased breakdown. Urinary iodine excretion was elevated in all groups on day 5 after iodine exposure. In group 1, which included 17 term newborns, the median TSH level was normal after 5 days and 2 weeks, only 1 case of transient hypothyrotropinemia was observed; T4 and T3 median levels were in the lower range of normal. In groups 2 and 3, which included 8 preterm infants of 15 newborns and 9 preterm infants of 19 newborns, respectively, the median TSH values were elevated and T4 and T3 levels were very low. Hypothyroidism was diagnosed in 6 of the 8 preterm and in 1 of the 7 term newborns of group 2. In group 3, 7 of the 9 preterm and 3 of the 10 term newborns reacted with hypothyroidism. Eight preterm and 3 term newborns had to be substituted with thyroxine. The thyroid function of term newborns was less affected by Amipaque or Omnipaque than by PVP-I. The data show that preterm infants are very sensitive to an iodine load.