Over a period of 2 years we have observed 3 thyrotoxic patients who presented with a convulsive encephalopathy. These patients had no history of seizures before and experienced no further seizures after subsidence of the thyroid dysfunction. During the convulsive period, electroencephalograms of 2 patients showed evidence of cerebral hyperexcitability, but in both cases it returned to normal once the thyroid disorder was corrected. We believe that thyrotoxic seizures are not rare. These seizures can be focal as well as generalized and can constitute the presenting symptoms of the disease. Absence of other laboratory abnormalities such as serum electrolytes, or osmolality changes, or hypoglycemia in all patients who reportedly suffered from thyrotoxic seizures, and the data obtained from animal studies, suggest that human thyrotoxic seizures result mainly from the direct effect of thyroid hormones over the cerebral tissue.