The high prevalence of co-occurring mental health and substance use disorders indicates a need for integrated behavioral health treatment that addresses both types of disorder simultaneously. One component of this integrated treatment is the use of an assessment process that can concurrently identify the presence of each class of disorder. The Addiction Severity Index (ASI) has been extensively used and researched in the field of substance use disorders for over 30 years. The ASI has seven sections, including a section on substance use disorders and a section on psychiatric symptoms, making it a potential candidate for a co-occurring screen during intake. The following study utilized a receiver operating characteristic curve analysis to determine an optimal cutoff score on the ASI psychiatric composite score to identify which individuals seeking substance use disorder treatment were admitted to the Milwaukee County Behavioral Health Division’s psychiatric hospital in the year subsequent to their ASI administration. Of the 19,320 individuals who completed an initial ASI in our system, 343 had an inpatient admission. The receiver operating characteristic curve was statistically significant, with an area under the curve of 0.75. A cutoff of 0.27 had a sensitivity of 0.77 and a specificity of 0.61, such that over 60% (11,963/19,320) of the sample was excluded. These results suggest that the ASI psychiatric composite score may be a useful initial screen to identify those with potential mental health problems/needs in a behavioral health system attempting to integrate addiction and mental health services.