AbstractExacerbation of psoriasis was observed in a 28‐year‐old patient following the addition of lithium carbonate to a treatment regimen of carbamazepine and other drugs for ***manic‐depressive psychosis. The biopsied lesion histologically showed a typical psoriasiform tissue reaction and immunohistochemically, remarkable infiltration of activated helper T cells. Immunologic reaction to lithium in a patch test and a lymphocyte proliferation test could not be demonstrated. The mechanisms of lithium action and their relations to the exacerbation of psoriasis were reviewed. In this patient, although we could not define which effect of lithium was most important, it seemed that an increased blood lithium level combined with simultaneous carbamazepine administration enhanced the triggering effect of lithium and caused the exacerbation of psor