The effect of pregnancy on the clinical course of Graves' disease was examined by studies on 41 pregnancies in 35 patients with Graves' disease, who were considered to be in a state of remission or near remission and were not receiving antithyroid drugs, during and after delivery. Eighteen of the 41 cases (44%) showed transient increases in the serum free T4 index (FT4 index) during weeks 10--15 of pregnancy, but normal thyroid function in the second and third trimesters. Similar transient increases in the serum free T3 index (FT3 index) were observed in early pregnancy in these patients. These early increases in the FT4 and FT3 indexes were specific to Graves' disease and were not observed in Hashimoto's disease. Two to 4 months postpartum, 32 cases (78%) developed various degrees of thyrotoxicosis, which was divided into 3 types: 1) persistent thyrotoxicosis with high radioactive iodine (RAIU) (10 cases), 2) transient thyrotoxicosis with normal or high RAIU) (10 cases), and 3) destruction-induced thyrotoxicosis with low RAIU (12 cases). An increase in the FT4 index in early pregnancy was significantly (P less than 0.001) associated with relapse of stimulation-induced thyrotoxicosis of either the persistent or transient type. Patients who developed destruction-induced thyrotoxicosis after delivery had significantly higher titers of antithyroid microsomal antibodies (P less than 0.001) and a longer euthyroid period before pregnancy (P less than 0.01) than patients who had recurrent persistent thyrotoxicosis. These data indicate that Graves' disease is aggravated in early pregnancy and after delivery and ameliorates in the latter half of pregnancy. Postpartum relapse of persistent hyperthyroidism could be predicted from an early increase in the FT4 index during pregnancy.