L-tri-iodothyronine was utilized in the treatment of myxedema psychosis. Psychopathology and metabolic state were carefully documented throughout the course of treatment. Part of the psychopathology observed was an organic brain syndrome related to the myxedema. Treatment with L-tri-iodothyronine resulted in disappearance of the organic brain syndrome and return to the euthyroid state. Persisting psychopathology appears related either to a premyxedematous depressive illness or to the response of the patient's personality to the myxedematous state. This segment of the psychopathology was responsive to chlorpromazine and electroconvulsive treatments.