Probation officers have large caseloads with high levels of psychiatric morbidity but receive minimal training in recognising/managing mental health problems. In the UK, there is no national screening procedure for mental illness among offenders that is considered effective. This study's aim was to develop a screening system for mental/personality disorders using the Offender Assessment System. Seven screens for mental disorder were developed using items from assessments on 574 prisoners: alcohol misuse, drug dependence, antisocial personality disorder, psychopathy, learning difficulties, psychosis and severe mood disturbance. Internal validation showed that the screens performed well across accuracy estimates. Prevalence of mental disorders identified with the screens in test and validation samples was similar. However, the psychopathy and severe mood disturbance screens may be more effective in screening for true negatives. The screens are an economical method for prioritising mental health needs of the UK prison and probation population using routinely collected data.