We found transient hyperthyroidism in the course of hydrocortisone withdrawal in two patients who had undergone unilateral adrenalectomy to resect cortisol-hypersecreting adenoma. A 38-yr-old woman showed clinical thyrotoxicosis 3 months after the operation. Serum T₄, T₃ and TBG levels were 11.9μg/dl, 310 ng/dl and 16.5μg/ml, respectively. She was given methimazole (MMI) mg/day for 4 weeks. After the cessation of MMI treatment, she eventually recovered to the euthyroid state. The other patient, a 34-yr-old man showed mild clinical symptoms of hyperthyroidism 2 months after the operation. Serum T₄, T₃ and TBG levels were 10.4 μg/dl, 240 ng/dl and 14.5μg/ml, respectively. In this case, no antithyroid drug was given. Two to three months the onset of hyperthyroidism, he returned to the euthyroid state spontaneously. We carefully eliminated the possibilty of factitious thyrotoxicosis in cases. They had neither neck pain nor fever. Both had low radioactive iodine uptake by the thyroid. Therefore, we diagnosed them as painless thyroiditis induced after the resection of hypersecreting adrenal adenoma.