Background: As in many European countries, Finnish psychiatric services experienced a rapid process of deinstitutionalization in the 1990s. In recent decades, the decrease in numbers of psychiatric hospital beds has in several countries been found to be linked with increasing criminality among severely mentally ill individuals. It has been concluded that deinstitutionalization could be the main reason for this development.

Aim: To investigate whether the prevalence of severely mentally ill persons to prison is a growing trend also in Finland.

Methods: We searched for the annual data in electronic case files of all prisoners about the ICD-10 diagnostic group of the most severe psychotic disorders. We also searched for the comorbid substance use disorders. Psychotic disorders due to substance abuse only were excluded from this study.

Results: During the years 2005-2016, a rapid 10-fold increase in psychotic disorders has occurred in Finnish prisons. In 2016, 185 prisoners (5.9% of all Finnish prisoners) were diagnosed as having a psychotic disorder that was not induced by substance abuse. The causes of this dramatic change are unclear. Comorbid substance abuse disorders were detected in only 39.5% of these prisoners and therefore substance abuse can only partly explain the increasing criminality of the severely mentally ill persons. 80.5% of psychotic disorders were diagnosed in Psychiatric Hospital for prisoners and 19.5% in prison policlinic services.

Conclusion: Trans-institutionalization has become a serious challenge for the Finnish criminal justice and mental health systems. Further studies are needed to detect the causes of this change. More specialized services for severely mentally ill persons are needed both in the community psychiatry and also inside the prison services.