To investigate relationships between hyperthyroidism and behavior, 45 formerly hyperthyroid patients (now euthyroid) and 51 control subjects were investigated by (a) a semi-structured psychiatric examination, (b) self-rating scales to assess mood states and personality, and (c) neuropsychological tests. Patients with "subclinical" or "remitted" hyperthyroidism showed more abnormalities than the controls in all dimensions investigated. Forty-three percent of patients (10% of controls) complained of "seriously reduced" well-being with feelings of fear, hostility, and inability to concentrate. While a fearful-agitated syndrome dominated in the initial phase of the illness, a mainly depressive syndrome was characteristic after a longer period of remission. More than 25% of the patients (2% of controls) showed "markedly impaired" neuropsychological functioning. Patients with a relapse within 2.5 years exhibited the most abnormal results. Even after a longer period of hormonal remission, there was no complete psychopathological and neuropsychological normalization. A thorough follow-up of hyperthyroid patients is recommended.