Objective:The Health of the Nation Outcome Scales (HoNOS) has been suggested as an outcome measure for the evaluation of mental health services in several countries; however, the scoring, reliability and structure of HoNOS are all ambiguous.Methods:A total of 80,161 completed HoNOS forms were checked for internal reliability and structural integrity. Parallel analysis was used to investigate the likelihood that HoNOS consists of 12 independent scales. The reliability of the scale was investigated using ordinal reliability analysis. Confirmatory factor analysis was used to investigate the factor structure.Results:Parallel analysis suggested that HoNOS had three underlying factors and could not be considered as 12 independent scales. The 12-item scale had good reliability. Confirmatory factor analysis found that a one factor, four-item scale produced the most acceptable fit.Conclusions:HoNOS should not be considered as 12 independent scales. Although the four-item scale is acceptable psychometrically, it has practical problems, as it focuses only on the social aspects of outcome measurement. It is possible that there are other factor structures that would encompass more items, but any study of these should test for invariance between groups of service users.