Previous efforts to define the case mix of psychiatric inpatients have generally relied on the use of psychiatric diagnosis, and have had little success elucidating the relationship between clinical outcome and resource utilization. The Severity of Psychiatric Illness (SPI) rating scale, a reliable chart-based system using nine clinical rating dimensions, has been developed by one of the authors for use in studying the case mix of psychiatric inpatients. This report is the first describing this scale. For a sample of 244 psychogeriatric patients, all but one of the dimensions were significantly correlated with at least one of the five baseline clinical assessments, and all but one of the SPI items were correlated with outcome assessment. Four dimensions of the SPI were related to clinical outcomes. The SPI also predicted length of stay (predicting 23% of the variance for patients discharged to their homes). This study provides strong evidence for the construct and predictive validity of the SPI among older adult psychiatric inpatients.