Three standardized screening instruments-the Global Appraisal of Individual Needs Short Screener (GSS), the Mini-International Neuropsychiatric Interview-Modified (MINI-M), and the Mental Health Screening Form (MHSF)-were compared to two shorter instruments, the 6-item Co-Occurring Disorders Screening Instrument for Mental Disorders (CODSI-MD) and the 3-item CODSI for Severe Mental Disorders (CODSI-SMD) for use with offenders in prison substance-abuse treatment programs. Results showed that the CODSI screening instruments were comparable to the longer instruments in overall accuracy and that all of the instruments performed reasonably well. The CODSI instruments showed sufficient value to justify their use in prison substance-abuse treatment programs and to warrant validation testing in other criminal justice populations and settings.