The authors review the medical literature and present two case histories regarding the efficacy of lithium carbonate in the treatment of chronic behavior disorders in mentally retarded adults. They conclude that the available data lend tentative support for the use of lithium to manage behavior disorders in retarded individuals which are characterized by lifelong hyperactivity, aggressiveness, and/or self-mutilation. They recommend that, when lithium is used for this indication, a blood level of approximately 1.0 mEq/1 is necessary, and that all patients be given a drug-free trial once a therapeutic effect has been achieved in order to assess the continued need for treatment. Etiological issues, a possible mechanism of action involving lithium's effect on CNS serotonergic activity, and areas for future research are discussed.