The purpose of this article is to report the prevalence of tardive dyskinesia using strict diagnostic criteria in a large sample of chronic mental patients. The criteria used were those of Villeneuve, and only patients with constant movements were counted. The disorder was significantly (p less than .001) less frequent in males (5.8 per cent) than females (10.6 per cent). The study found an overall prevalence of 8.2 per cent in 3140 patients. This figure should represent a minimal prevalence figure since only patients with severe symptoms were included.