The source literature points to the increasing number of crimes committed by people with mental diseases. Based on international data, it is estimated that the incidence of mental disorders among inmates is increasing at a disproportionate rate in prisons around the world. It is estimated that even every fourth convict may have a diagnosis of mental illness. It is worth noting that repeat offence rates among mentally ill people are also higher than in the general population of repeat offenders. The repeat offence rate among offenders with a diagnosis of mental illness ranges from 60% to 80%. This paper is intended to describe the situation of people diagnosed with mental illness in Poland, with reference to the available statistical data and international research.