We report a new type of thyrotoxicosis (postaspiration thyrotoxicosis) that occurred after needle aspiration of thyroid cysts. In a retrospective study five patients with thyroid cysts developed transient thyrotoxicosis without significant symptoms of hyperthyroidism after needle aspiration. These patients had elevated serum thyroid hormone levels with suppressed serum TSH and thyroid radioiodine uptake. The thyroid hormone content in the cystic fluid was greater in patients who developed thyrotoxicosis than in patients who did not. In a prospective study we examined thyroid function tests after needle aspiration in 52 consecutive patients with thyroid cysts and 63 patients with solid thyroid nodules and confirmed an occurrence of postaspiration thyrotoxicosis in a patient with thyroid cyst. Our observation indicates that postaspiration thyrotoxicosis occurs only in patients with thyroid cyst and that the incidence of this disorder is probably less than 1% of patients who undergo thyroid needle aspiration. Although the exact mechanism of elevated serum hormone levels is unknown, an inflammatory process after needle aspiration might have triggered the release of thyroid hormone.