Tardive dyskinesia constitute the most serious complications of long term treatment with neuroleptics. In this study e,140 patients (1,568 men, 1,572 women) were examined in 4 psychiatric hospitals. 258 (8.2%) patients were found to present such a syndrom. The frequency of tardive dyskinesia was higher among female patients (167 women; 10.6%) than among male patients (91; 5.8%). On the average dyskinetic women were older than men (64.9 versus 54 years old). In 59.3% of the cases, dyskinesia was limited to an oro-facial symptomatology, but in 40.6% of the cases, this was associated with a choreo-athetoïd syndrom. Most of these patients were chronic patients. On the average they had been in hospital for 20 years, and had received neuroleptic treatment for 15 years. In half of the patients, the treatment was undergone for schizophrenia and chronic delusion. Other syndroms encountered were mental retardation, dementia, affective disorders, epilepsy, alcoholism, character disorders, etc... The frequency of tardive dyskinesia that we observed is certainly a minimum since there exits a number of transient pathological forms. We discuss a few aspects of treatments of tardive dyskinesia with neuroleptics, lithium, diethylaminoethanol, choline... we insist particularly on the problem of prevention. In patients treated with neuroleptics, tardive dyskinesea might be stopped at an early stage: secondary prevention, consisting in an early detection of the first symptoms of lingual or facial dyskinesia, should lead, if possible, to the withdrawal of the treatment. Primary prevention appears to be very important: the risk of drug-induced dyskinesia appears to be higher among people above 50 of age, and neuroleptic treatment should be administered only for the most serious forms of mental illnesses.