Thirty-five pregnant women with thyroid antibodies were followed during pregnancy and 12 month post partum. Twenty antibody-negative women served as controls and none of these developed postpartum thyroiditis in contrast to 12 of 35 antibody-positive women. Umbilical cord blood was collected at birth for measurements of thyroid antibodies, and blood samples for measurements of thyroid hormones were obtained at 5 and 30 days of age in the infants together with a clinical examination. There were no differences between the infants of antibody-positive and -negative mothers or the infants of those who developed postpartum thyroiditis with regard to gestational age at birth, birth weight, birth length or Apgar score, and no difference in thyroid function of the mothers and infants. However, serum TSH level was significantly higher in mothers who subsequently developed postpartum thyroiditis. Furthermore, a significantly lower growth during the first 30 days of life was observed in their infants. Our observations suggest a relationship between the maternal thyroid status in pregnancy and early growth in infancy.