Objective-To clarify the usefulness, acceptability, sensitivity, and validity of version 4 of the Health of the Nation Outcome Scale (HoNOS), a scale developed to meet the requirement for a clinically acceptable outcome scale for routine use in mental illness services. Design-Patients with a range of mental illnesses were rated on the HoNOS at the beginning and end of an episode by interviews with mental health professionals. Subjects-934 patients from eight diagnostic categories were rated by 129 mental health professionals at 17 sites; 250 were also rated on a range of comparison scales. Outcome measures-Comparison of patients' scores at the beginning and end of an episode using individual item scores, dimensional subscores, and the total score. Results-HoNOS scores decreased by almost 50% between the beginning and end of episodes. They varied with the severity of the setting and discriminant analysis showed that the HoNOS had a moderate level of discriminatory power. Correlation analysis showed acceptable levels of agreement with independent scales, although the accuracy of ratings of some items at the beginning of an episode was aVected by information deficits. Conclusion-The findings indicate thatHoNOS is sensitive to change across time and to diVerences in illness type and severity, and has a suYcient degree of both construct and criterion related validity to fulfil the requirements of a mental health outcome scale for routine use in clinical settings.