Data regarding the incidence and effect of maternal thyroid antibodies on neonatal thyroid function are conflicting. In order to elucidate this aspect, antimicrosomal thyroid antibodies were measured: (1) in cord serum of a normal populations, (2) in the eluate of blood spots of infants with normal filter paper spot T4 and TSH, (3) in the eluate of blood spots from our recalled population (low T4 and normal TSH), and (4) in serum of detected hypothyroid infants. The incidence of MCA with titer greater than 1/40 in cord sera was 8% (115 of 1.383). There was no statistical difference in cord serum T4' T3' or TSH concentrations in these newborn infants compared to the MCA negative population. In 1,000 spots with normal T4 and TSH, 11 or 1.1% were positive for MCA. In 1,630 spots with low T4' 18 positive cases were discovered, or 1.1%. There was excellent correlation between maternal MCA titers and newborn infant titers either in sera or spots when paired samples were available. Finally, only one of 104 detected infants with primary hypothyroidism had detectable MCA. These results indicate a high incidence (8%) of MCA in our presumed normal newborn population, MCA does not decrease serum T4' T3' and TSH concentration or filter paper spot T4 and TSH, and thyroid autoimmunity is not a frequent cause of congenital hypothyroidism.