The frequency of autoantibodies to thyroid microsomes (MAb), thyroid peroxidase (TPOAb) and thyro-globulin (TgAb) was studied in 736 women during and after pregnancy. The aim was to study the relationship between TPO Ab and post-partum thyroid disease, and to compare their behaviour with MAb. Seventy-five (10%) were either MAb or TgAB positive, of whom 36 were sampled serially at 18, 30 and 36 weeks of pregnancy and 1, 6 and 12 months post partum. Twenty of the antibody negative mothers were selected at random for controls. Twelve of the 36 antibody positive mothers developed post-partum thyroid dysfunction (group I), 24 did not (group 2) compared with none of the 20 controls (group 3). Six months post partum. TPO Ab titres rose overall in both groups I and 2, but only in those with measurable TPO Ab during the first tremester. A TPO Ab was less frequently positive than MAb, but the difference was not statistically significant. TPO Ab are not superior to MAb for predicting post partum thyroid disease in pregnant women.