To quantify the impact of chronic exposure to neuroleptic medication on the occurrence of tardive dyskinesia (TD), we conducted a meta-analysis of data collected from 21 studies published between 1966 and 1985. The observed prevalence of dyskinesia was greater among exposed subjects in all 21 studies; we estimate that, on the average, the occurrence rate was 2.9 times greater in exposed persons than would be expected if they had been unexposed. We estimate that 65 per cent of exposed cases and 51 per cent of all cases in these investigations were caused by long-term neuroleptic exposure. Among adult United States residents in 1980, we estimate that there were approximately 193,000 neuroleptic-induced TD cases of which about 60 per cent occurred in outpatients. We also observed substantial heterogeneity of effect (rate ratio) across studies, however, partially explained, by changes and differences in the rate of dyskinesia, by differences in the frequency of certain effect modifiers, and by differences in diagnostic methods. Methodologic limitations of the studies and their possible effects on our results are discussed.