Objective: Assessment of functional status is increasingly important in clinical trials and outcome research. Although several scales for assessing functioning are widely used, they vary in coverage, and direct comparisons among them are rare. Comparative information is useful in guiding selection of appropriate scales for research applications. Method: Results from three scales that measure functioning - the Medical Outcomes Study 36-item Short-Form Health Survey, the Social Adjustment Scale Self-Report, and the Social Adaptation Self-Evaluation Scale - were compared in a consecutively selected sample of 211 patients coming to primary care. Patients also received psychiatric assessments. Results: All three scales were acceptable to patients, showed few significant correlations with demographic variables, and were able to differentiate psychiatrically ill and well patients. Correlations among scales, even among scale items that assessed similar domains of functioning, were modest. Conclusions: Although all three scales are presumed to assess functional status, their item content and coverage differ. Selection of a scale requires a review of the scale items and consideration of research priorities and the characteristics of the study group. If functional status is a critical outcome measure, use of more than one scale may be necessary.