OBJECTIVE: Given the prevalence of substance abuse among persons with psychiatric disorders, substance use assessment should be an integral component of mental health evaluations. This study examined the validity of a set of two 5-point rating scales developed for use by mental health clinicians in rating individual clients' levels of alcohol and other drug use. METHODS: A sample of 116 psychiatric outpatients who were participating in a study of psychosocial functioning and substance use was assessed by researchers using an extensive battery of instruments that included the Addiction Severity Index and the Timeline Follow-Back interview. Each client's primary therapist completed the 5-point rating scales to indicate the client's levels of alcohol and drug use. RESULTS: Clients were grouped according to their ratings on the 5-point scales. Significant differences between the groups were found on self-reported patterns of current alcohol and drug use and substance use history. CONCLUSIONS: Outpatient therapists provided ratings of clients' alcohol and drug use that corresponded well with substance use data obtained from an extensive research battery. The study results support use of clinician rating scales as a screening tool for identifying problematic alcohol and other drug use among psychiatric outpatients.