BACKGROUND:Though slowly growing, knowledge about prisoners detained for having violated an Alien Act is still marginal and most studies involve detained asylum seekers in the USA and Australia. Little is known about prevalence rates of mental health disorders in such a population. The Brief Jail Mental Health Screening BJMHS has been demonstrated in other prison populations as a valid screening for serious mental illness. AIM:The aims of this study were to describe prevalence rates for mental disorders according to ICD-10 and to validate the BJMHS for this population. METHODS:80 inmates at a detention center for prisoners having violated the Swiss Aliens Act were surveyed using the BJMHS at their admission. The results were cross validated with the WHO Composite International Diagnostic Interview (CIDI). RESULTS:When omitting disorders caused by smoking tobacco, 76% of the prisoners suffered from at least one mental disorder according to CIDI. Whereas the rates for disorders due to psychoactive substance use as well as schizophrenic and affective disorders were comparable with other prison populations, we found a specific increased reporting of phobic (14%) and post-traumatic stress disorders (23%). The BJMHS detected serious mental illness defined as schizophrenic or affective disorders with a sensitivity of 81.0% and a specificity of 74.6%. DISCUSSION:As in other prison populations prevalence rates for mental disorders were markedly above the general population. The specific pattern with high rates of phobic as well as post-traumatic stress disorders may reflect the very often traumatic backgrounds of this population. Whereas the results for the validation of the BJMHS were even better than in other similar studies and the instrument proved to be practicable and helpful to detect serious mental illness, sensitivity for a screening tool of around 80% is still too low. Additionally the fact that other serious mental disorders are not covered emphasizes the importance of other elements in the screening process, including the need to have well-trained staff, and to have a low threshold for psychiatric examination.