Using the method of concurrent radioisotope in-vitro analysis the authors examined the serum levels of thyroid hormones in 70 patients with temporal epilepsy. As a result of the examination two groups of the cases could be differentiated. In 53 patients the hormonal activity of the thyroid appeared to be lowered, while in 17 patients it was increased. A certain relationship between the clinical manifestations and course of the epilepsy on the one hand, and thyroid function on the other was revealed. More unfavourable variants of the epilepsy were found to be associated with subclinical hypothyrosis. The following phenomena are assumed to be criteria of the unfavourable course of epilepsy: a greater frequency of general convulsive paroxysms and epileptic states, early development and steady progress of the intellectual and emotional impairments; more marked and diffuse disorganization of the brain bioelectrical activity, and loss of the elastic properties by cerebral vessels.