Abstract: Background: The prevalence of psychotic disorders among prisoners is relatively high. We sought to investigate the prevalence of men who have a very high risk of developing psychosis in a prison population.Methods: The Prodromal Questionnaire — Brief Version (Loewy, Pearson, Vinogradov, Bearden and Cannon, 2011), was used to screen newly-arrived prisoners in a London prison for features associated with an increased risk of psychosis. Concurrent validity was evaluated using the Comprehensive Assessment for At Risk Mental State (Yung et al., 2005).Results: 750 prisoners were screened and 301 were underwent further clinical assessment. 5% the total number of those screened met diagnostic criteria for the ARMS and 3% had recently developed a first episode of psychosis. Using endorsement of items that also caused distress, the PQ-B predicted an ARMS or a psychotic disorder with 90% sensitivity and 44% specificity.Conclusions: The PQ-B is effective in identifying people who are vulnerable to developing psychosis in a prison population.