Of the three standard instruments tested, the Mental Health Screening Form (MHSF) produced the highest overall accuracy (73 percent) and the best sensitivity (90 percent). In summary, the MHSF was the most consistently accurate instrument under the circumstances of the study and was best able to identify any mental disorder when present. The six-item Co-Occurring Disorders Screening Instrument for Mental Disorder (CODSI-MD) developed from items selected from the standard screeners, demonstrated promise as a potential brief screening instrument. The development of a screening device to identify severe mental disorders, the Co-Occurring Disorders Screening Instrument for Severe Mental Disorders (CODSI-SMD) was a secondary goal that emerged in the course of conducting the study. Offenders who have co-occurring substance use and mental disorders constitute a population of significant concern. However, this rapidly growing population remains largely understudied. The purpose of this study was to identify an instrument and cutoff scores that best meet the needs of criminal justice systems to screen for mental disorders in prison substance abuse treatment settings. Tables, figure, references