Objective: The researchers' aim was to evaluate the construct validity of a rating scale used by case managers to assess the clinical status of clients in a large out-patient treatment program for persons with severe mental illness and concurrent drug or alcohol problems. Methods: A standardized rating scale for assessing clients' levels of four key clinical problems—psychiatric symptoms, substance use, treatment noncompliance, and overall level of dysfunction—was developed for use by case managers. After being trained in use of the instrument, 20 case managers rated 302 clients along these dimensions. Case manager ratings were compared with data on clients' diagnoses and the level of structure they required in their treatment program. Results: About 42 percent of the clients assessed had schizophrenia, 28 percent had bipolar disorder, 14 percent had major depression, and 11 percent had other psychotic disorders. The case manager ratings were significantly related to clients' diagnosis and the level of structure in their treatment program. The four dimensions were also correlated with each other; strong correlations were found between severity of substance use and noncompliance and between severity of psychiatric disorder and level of dysfunction. Conclusions: Ratings by case managers can be used to measure individual progress in treatment as well as the status of the entire client population in a treatment program.