Objective: This study aimed to assess the validity of the psychiatric problems subscale of the Addiction Severity Index ASI psych to ascertain psychiatric comorbidity among individuals participating in randomized controlled trials RCTs of substance use disorder SUD treatments.Methods: The ASI psych score among 1,660 RCT participants of National Institute of Drug Abuse Clinical Trials Network studies was compared against diagnosis of any serious mental disorder based on the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders Fourth Edition SCID or Mini International Neuropsychiatric Interview MINI . Sensitivity, specificity, positive predictive value PPV , negative predictive value NPV , and area under the curve AUC for detecting any serious mental disorders were estimated by the receiver operating characteristic ROC analysis.Results: Based on the overall sample, the AUC score for any serious mental disorder was 0.72 95 confidence interval CI , 0.69, 0.75 with the optimal ASI psych score of 24.6. ...