Growing pressure on mental health care providers and managed care organizations to document the effectiveness of their treatments requires linking improvements in the process of care with measurement of the outcomes of care. Outcomes assessment in psychiatry has previously focused mainly upon specific conditions (such as anxiety disorders or depression). OBJECTIVE: We sought to develop a scale that would be applicable across settings, treatments, and patient diagnoses, and would be meaningful to clinicians of different theoretical persuasions. Using 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. METHODS: Item development for the scale was guided by literature review, and interviews with senior clinicians and patients. We used three different samples of inpatients, outpatients, and non-patients for item reduction, reliability analysis and validation (total n = 257). RESULTS: We reduced our initial item pool from 81 to 10 items. The 10-item scale had high internal consistency (Cronbach’s ? = .96), and showed strong correlations with commonly-used measures of psychological well-being and distress. The Rasch characteristics of the final 10-item scale were also good. The sample (subject) separation was 4.2, the sample ? was .94, the item separation was 3.0 and the item ? was .90. CONCLUSIONS: Our results suggest that the scale appears to measure a broad domain of psychological functioning. Moreover, the scale appears to lack ceiling and floor effects, and discriminated between inpatients, outpatients, and non-patients, suggesting the scale has excellent potential to be broadly responsive to a variety of treatment effects.