Objective This paper tests the validity of a simple and easy to use scale, the Modified Mini Screen (MMS) to identify people with mental health problems in the treatment ecology for chemical dependency, including specialty sector substance abuse treatment settings, and shelters, jails and street community outreach programs. Methods Four hundred and seventy-six individuals in chemical dependency treatment and in jails, shelters and outreach settings completed a set of 22 items to screen for mental disorders-the MMS-and a validation interview-the Structured Clinical Interview for Diagnosis (SCID). ROC curve analysis was used to (1) determine the optimal range of cut points for identifying mental health problems in this culturally heterogeneous sample and wide range of treatment and community settings, and (2) calculate overall accuracy, sensitivity, specificity, and positive and negative predictive values for the MMS. Results Forty three percent of the sample met criteria for a DSM-IV diagnosis of anxiety, mood or psychotic disorder. At cut points of 6-9, the sensitivity of the MMS ranged from 0.63-0.82, its specificity ranged from 0.61-0.83, and its overall accuracy ranged from 70-75%. At these cut points, the MMS screen performed equally well for men and women, and for African Americans and Caucasians, and is comparable to the performance of other screens validated for less compromised populations. A decision tool has been developed based on these results that is available from the authors.