Serum antidouble stranded DNA antibody levels were measured during pregnancy and after delivery in women who had postpartum exacerbations of Graves' and Hashimoto's diseases. The changes in serum anti-DNA antibody levels closely paralleled those in the serum free T4 index, significantly increasing in the thyrotoxic phase 3-8 months postpartum in women with postpartum exacerbations of thyrotoxicosis due to Graves' disease and 1-3 months postpartum in women with postpartum destructive thyrotoxicosis of Graves' and Hashimoto's diseases. No change in anti-DNA antibody level was found in women with no postpartum exacerbations of thyroid diseases, nor could we demonstrate significant increases in serum anti-DNA antibody titers in patients with thyrotoxicosis due to subacute thyroiditis or in normal pregnant and postpartum women. The changes in serum anti-DNA antibody titers may reflect some generalized immunological abnormality in women who have postpartum exacerbations of Graves' or Hashimoto's diseases.