BACKGROUND Anaplastic thyroid carcinoma (ATC) is one of the most aggressive human malignancies and constitutes approximately 1.6-5% of the malignant neoplasms of the thyroid gland. ATC usually manifests itself with the local symptoms due to a rapidly enlarging thyroid mass, and as other thyroid cancers, has only seldom been reported to cause thyrotoxicosis. Up to now only 9 cases of ATC with concomitant thyrotoxicosis have been described. METHODS We report a rare case of a 66-year-old woman, who had had the preexisting large, euthyroid multinodular goiter for almost 50 years. She was consulted by a doctor because of a 4-week history of thyrotoxicosis, symptoms of the congestive heart failure and a rapid increase in the size of the goiter. Thyroid hormone levels were consistent with a hyperthyroid state. The fine-needle aspiration biopsy confirmed a diagnosis of the anaplastic thyroid carcinoma, the small cells variant. The 99m Tc-pertechnetate scintigraphy visualized non-homogenous tracer distribution with hot nodules. She was given a doxorubicin (20 mg/week) and required the continuous antithyroid treatment. The patient died a one year after the first symptoms of the disease occurred. CONCLUSIONS The association between ATC and a thyrotoxic state is very rare. In most cases, thyrotoxicosis concomitant with ATC was thought to be a result of the destruction of the thyroid follicles by the rapid infiltration with malignant cells, resulting in the leakage of preformed hormones to the circulation. In that case the most probable cause of thyrotoxicosis was the multinodular goiter coexisting with ATC. A simultaneous onset of tumor growth, thyrotoxicosis and a relatively long survival time of our patient is worth to notice and discuss.