Assays for the thyroid-stimulating antibody (TSAb) of Graves' disease were performed with serum obtained from 17 women with 20 pregnancies who either had or had had Graves' disease, or who had delivered a child with neonatal hyperthyroidism. Ten of the children, of eight women, were diagnosed as being hyperthyroid; all eight mothers had high concentrations of TSAb, as measured by adenylate cyclase stimulation. In the infants with neonatal hyperthyroidism, a minimum 500% increase in cAMP was found on assay of maternal immunoglobulin G, and no such high values were associated with a euthyroid infant. Blood samples for TSAb assay were available from 11 mothers on both the day of delivery and at least 1 other time in the pre- or postpartum period. In 7 mothers, the lowest value (negative in 4) was obtained coincident with delivery, and in the remaining 4, there was no pregnancy-associated change. Thus, a pattern, related to pregnancy, of a decline in TSAb concentration or a subsequent postpartum increase was observed in the majority of subjects. Apparently, neonatal hyperthyroidism due to transplacental passage of TSAb occurred only when this decline did not reduce the concentration to a low value.