OBJECTIVE: To assess the feasibility and measurement characteristics of ratings completed by professional associates to evaluate the performance of practicing physicians. DESIGN: The clinical performance of physicians was evaluated using written questionnaires mailed to professional associates (physicians and nurses). Physician-associates were randomly selected from lists provided by both the subjects and medical supervisors, and detailed information was collected concerning the professional and social relationships between the associate and the subject. Responses were analyzed to determine factors that affect ratings and measurement characteristics of peer ratings. SETTING AND PARTICIPANTS: Physician-subjects were selected from among practicing internists in New York, New Jersey, and Pennsylvania who received American Board of Internal Medicine certification 5 to 15 years previously. MAIN OUTCOME MEASURE: Physician performance as assessed by peers. RESULTS: Peer ratings are not biased substantially by the method of selection of the peers or the relationship between the rater and the subject. Factor analyses suggest a two-dimensional conceptualization of clinical skills: one factor represents cognitive and clinical management skills and the other factor represents humanistic qualities and management of psychosocial aspects of illness. Ratings from 11 peer physicians are needed to provide a reliable assessment in these two areas. CONCLUSIONS: These findings suggest that it is feasible to obtain assessments from professional associates of practicing physicians in areas such as clinical skills, humanistic qualities, and communication skills. Using a shorter version of the questionnaire used in this study, peer ratings provide a practical method to assess clinical performance in areas such as humanistic qualities and communication skills that are difficult to assess with other measures.