Fourteen severe mentally retarded aggressive adolescents were administered with lithium for 8 months. The daily dosages of lithium were 6-48 mEq/patient, and the serum lithium level ranged between 0.30-0.95 mEq/1. Lithium seems to have a significant affect-damping and antiaggressive effect in mentally defective male adolescents who had been previously resistant to phenothiazines. Excessive polydipsia connected with highly undisciplined fluid intake and polyuria connected with frequent bed-wetting, however, represented a considerable limitation of the use of lithium in these states.