Using a combination of classical test theory and Rasch item analysis, we developed a short scale designed to measure the effectiveness of mental health treatment across a wide range of mental health services and populations. Item development for the scale was guided by literature review and interviews with senior clinicians and with patients. Using 3 different samples consisting of inpatients, outpatients, and nonpatients, we reduced our initial item pool from 81 to 10 items. The 10-item scale had an alpha of .96 and showed strong correlations with commonly used measures of psychological well-being and distress. Our results suggest that the scale appears to measure a broad domain of psychological health. The scale appeared to lack ceiling and floor effects, and it discriminated between inpatients, outpatients, and nonpatients, suggesting the scale has excellent potential to be broadly responsive to a variety of treatment effects. In addition, the new scale proved to be sensitive to treatment changes in a sample of 20 psychiatric inpatients. Overall, the initial data suggest that we have developed a brief, sensitive outcome measure designed to have wide application across psychiatric and psychological treatments and populations.