In France, the prevalence of psychiatric disorders has never been studied, despite the fact that such disorders are a major public health problem in prisons. Through the Services Medico-Psychologiques Regionaux (SMPR, Regional Medical and Psychological Departments), the psychiatric disorders of new prisoners, as well as the mental pathologies of the prisoners monitored by these services, have been analysed. All subjects arriving in prison during the month of June 2001, as well as all those monitored during the same period, were surveyed by means of a questionnaire that assessed the symptoms present on arrival in prison and the mental pathologies of the prisoners being monitored. Most of the new prisoners were male (94%). They were relatively young (31 years on average) and had severe problems with social insertion (12% were homeless). There were psychiatric symptoms in 40%, largely disorders associated with addiction or anxiety. The prisoners monitored by the SMPR were older and included more women, but the socio-economic characteristics were similar to those of the new arrivals. Personality disorders represented 34% of the diagnoses, a quarter of the prisoners being monitored suffered from mental disorders associated with the use of toxic substances, 12% suffered from neurotic or anxiety disorders, 8% had a psychotic pathology and 7% mood disorders. Psychiatric disorders are very common in new prisoners and the mental pathology of the subjects monitored by the SMPR is very specific.