We carried out a close follow-up study of 96 episodes of postpartum hyperthyroidism, which developed in women with a history of Graves' disease. Hyperthyroidism had developed 1-6 months (mean +/- SD, 2.3 +/- 1.4) after delivery. Radioiodine uptake (RAIU) values during the thyrotoxic phase, determined after a 1-week restriction of iodine-rich diet, ranged from very low to very high (1-70%). The group with low RAIU values (group L: < 10%, n = 26) and the normal group (group N: 10-40%, n = 33) showed significantly higher urinary excretion of iodine (UI) than the high group (group H: > 40%, n = 37), and UI in group L was comparable to that in patients with silent thyroiditis. TSH binding inhibiting antibody (TBIAb) values were determined in 87 patients at the RAIU testing, and were above normal in most of them: all in group H, 68% in group N, and 52% in group L. The mean TBIAb value in group H was significantly higher than that in group N or L (P < 0.0001 for both). Fifty-one patients in group L and group N were observed without treatment. Of these, hyperthyroidism resolved spontaneously in 39 patients (76%), in whom transient hypothyroidism developed with substantial frequency. Hyperthyroidism subsequently resumed in 18 (46%) of these 39 between 4 and 9 months (mean +/- SD, 7.1 +/- 2.1) after delivery and did not resume in the other 21. RAIU values, determined again during this later phase of hyperthyroidism, were high enough to indicate Graves' hyperthyroidism in all but one patient. During the postpartum period, TBIAb values increased and then declined in the majority of the patients irrespective of the course of hyperthyroidism. Of the 21 patients who maintained euthyroidism after spontaneous resolution of hyperthyroidism, transient increase in TBIAb was observed in 14. These findings suggest that silent thyroiditis commonly develops concomitantly with the activation of Graves' disease and delays or masks the development of Graves' hyperthyroidism.