Levels of mental illness amongst prisoners are high, but little is known about the situation of those serving community sentences. More information is needed at a time when the UK Department of Health is seeking to improve mental health service access for all offenders. To estimate the prevalence of both current and lifetime mental illness, an epidemiological survey was conducted on individuals under probation supervision in Lincolnshire, England. Assessment included the Mini International Neuropsychiatric Inventory and other measures of mental health. We estimate that around 39% of individuals in this probation population are suffering from current mental illnesses, anxiety disorders being the most common. Furthermore, around 60% have substance abuse problems and around 48% have personality disorders. Lifetime mental illness, co-morbidity and dual diagnosis were also found to be high. In common with other groups of offenders, the prevalence of mental health problems in those serving community sentences is high.