Long-term chlorpromazine therapy has been associated with the asymptomatic development of a high incidence of antinuclear antibodies, coagulation inhibitors, and increased serum levels of IgM. The purpose of this study has been to characterize the natural history of this chlorpromazine-induced (CPZ) immunopathy. To this end we carried out a prospective study of schizophrenic patients with the immunopathy to compare the effect of continuing CPZ versus switching to haloperidol therapy. Although no marked differences were noted between the 2 groups at the end of 5 years, 6 of 29 patients who continued to receive CPZ, as compared to none of 14 patients on haloperidol, had progressive elevations of serum IgM. In spite of a high incidence of antinuclear antibodies, none of the patients developed a lupus-like syndrome. One patient, however, who had been maintained on CPZ for more than 15 years, developed Waldenström macroglobulinemia, as characterized by an IgM monoclonal gammopathy and a lymphocyte immunoglobulin heavy and kappa light chain gene rearrangement. Another CPZ-treated patient developed immune thrombocytopenia. Based on the potential serious sequelae of prolonged stimulation of the immune system by CPZ, we recommend that patients who develop an increase in serum IgM while on CPZ be switched to other types of anti-psychotic medications.