The thyroid function of very-low-birthweight (VLBW; below 1500 g) infants admitted to neonatal intensive-care units was studied at two hospitals; one routinely used topical iodinated antiseptic agents and the other used chlorhexidine-containing antiseptics. Serial monitoring of urinary iodine excretion and serum thyrotropin and thyroxine levels was undertaken from birth for the first 4 weeks of life. Urinary iodine excretion rose dramatically in the 54 iodine-exposed infants and was up to fifty times greater than in the 29 non-exposed infants. Within 14 days, 25% (9 of 36) of the infants exposed to iodine had serum thyrotropin levels above 20 mIU/l, compared with none of the control group. The mean serum thyroxine level in these 9 infants (44.1 nmol/l) was significantly lower than that in exposed infants with normal thyrotropin levels (83.1 nmol/l) and in the non-exposed control group (83.0 nmol/l); thyroxine levels fell before serum thyrotropin rose. These disturbances in thyroid function correlated positively with urinary iodine excretion and hence iodine absorption. Thyroid function had returned to normal by the time of discharge from hospital. It is concluded that iodine absorption, from topical iodine-containing antiseptics, may cause hypothyroidism during a critical period of neurological development in the newborn infant. The routine use of iodine antisepsis in VLBW infants should be avoided because of this effect.