The transplacental transfer of maternal antithyroid antibodies has recently been hypothesised as an aetiological factor in CH. In order to test this hypothesis, mothers and newborns identified by neonatal screening as suffering from hypothyroidism were tested for TgAb, MAb and TSHBAb. Significant titres of MAb and TgAb antibodies were found in 5% of the newborns and their mothers. TSHBAb was found in 1 out of 18 newborns and 1 out of 14 mothers. A causal link was found between the transplacental transfer of IgG inhibiting thyroid growth and function induced by TSH from a mother with Hashimoto's thyroiditis a newborn with CH and in her CH newborn. However the present series did not reveal the high percentage of cases with CH and thyroid antibodies reported by others and particularly not among the neonates born to mothers without any kind of maternal thyroid pathology. It therefore seems that the role of autoimmunity in the pathogenesis of CH and TH is yet to be clarified.