A 35-year-old hyperthyroid woman who developed nausea, vomiting, tachycardia, nystagmus and mental disturbance, was referred to our hospital with a suspected diagnosis of thyroid storm. However, the thyroid gland was only slightly palpable, bruits were not audible, and exophthalmos was not present. Serum levels of thyroid hormone were increased, but TSH receptor antibodies were negative. Echography and color flow doppler ultrasonography revealed a slightly enlarged thyroid gland and a slightly increased blood flow, both of which were much less milder than those expected for severe hyperthyroid Graves' disease. Under the diagnosis of hyperthyroidism due to gestational thyrotoxicosis associated with Wernicke encephalopathy, vitamin B1 was administered on the first day of admission. Her consciousness became nearly normal on the second day except for slight amnesia. Her right abducent nerve palsy rapidly improved, but horizontal and vertical nystagmus, diminished deep tendon reflexes and gait ataxia improved only gradually. MRI findings of the brain were compatible with acute Wernicke encephalopathy. We concluded that history taking and physical findings are important to make a differential diagnosis of gestational thyrotoxicosis with acute Wernicke encephalopathy from Graves' thyroid storm, and that Wernicke encephalopathy should be treated as soon as possible to improve the prognosis.