This article describes the development of an instrument to screen male and female offenders for co-occurring substance use and mental disorders. This phase developed and pilot tested (N = 100) the Criminal Justice Drug Abuse Treatment Studies (CJDATS) Co-occurring Disorders Screening Instrument for Mental Disorders (CODSI-MD), a 6-item instrument derived from three standard mental health screeners. The overall accuracy of the CODSI-MD (81%) compared favorably with the three standard instruments. A second 3-item instrument, developed to screen for severe mental disorders (the CODSI-SMD), had an overall accuracy of 82%. The results of this pilot study must be viewed cautiously, pending validation of the findings with a larger sample.