This study is concerned with the long-term effects of lithium on recurrent affective disorders. Therapeutic, biological and adverse responses are reported. Evidence points to therapeutic efficacy in patients with bipolar mood disorders. Therapeutic action appears to be reflected in a reduction of mood fluctuations resulting in lower frequency of hospitalizations. Some theoretical and clinical issues remain unsolved. Among these is the assumption of prophylaxis. Presently, evidence points to a compensatory mode of action which does not eradicate, but counteract those symptom manifestations which accounted for higher frequency of hospitalization in the prelithium era.