Soon after the introduction of antipsychotic drugs into clinical practice, these agents were observed to be capable of producing not only acute extrapyramidal ("parkinsonian") side effects, but also later occurring abnormal involuntary movements that came to be called tardive dyskinesia. Since antipsychotic drugs are used in a variety of conditions that include psychotic features, studies have attempted to determine whether specific diagnostic subgroups may experience different degrees of vulnerability to drug-induced movement disorders. This issue is important not only to inform clinical practice, but also to provide clues to pathophysiology. A number of studies suggest that patients with affective disorders are at greater risk for developing tardive dyskinesia (controlling, to the extent possible, for other relevant variables such as age, sex, length of treatment). Encouraging preliminary data with new antipsychotic drugs such as olanzapine suggest that the risk of tardive dyskinesia associated with long-term antipsychotic drug use may be substantially reduced. This would go a long way toward improving the benefit-to-risk ratio of antipsychotic drug treatment, particularly in patients with affective disorders.