Thyroid antibodies were measured sequentially in 25 pregnant women from a Sri Lankan population. A high prevalence of antithyroid antibodies, particularly antithyroglobulin antibodies (TgAb) had previously been demonstrated in female schoolchildren drawn from this population. In the present study TgAb were detected in 36.8% of nonpregnant controls while thyroid peroxidase antibody (TPOAb) positivity was present in 26.3%. The prevalence of both antibodies in the pregnancy study group showed a progressive decline compared to nonpregnant controls throughout gestation becoming undetectable in the third trimester. The results are consistent with an immunosuppressive effect of pregnancy in a population in whom high thyroid autoantibody titers may have resulted from a recent salt iodization program.