BACKGROUND The College of Physicians and Surgeons of Ontario, the regulatory authority for physicians in Ontario, Canada, conducts peer assessments of physicians' practices as part of a broad quality assurance program. Outcomes are summarized as a single score and there is no differentiation between performance in various aspects of care. In this study we test the hypothesis that physician performance is multidimensional and that dimensions can be defined in terms of physician-patient encounters. METHODS Peer assessment data from 532 randomly selected family practitioners were analyzed using factor analysis to assess the dimensional structure of performance. Content validity was confirmed through consultation sessions with 130 physicians. Multiple-item measures were constructed for each dimension and reliability calculated. Analysis of variance determined the extent to which multiple-item measure scores would vary across peer assessment outcomes. RESULTS Six performance dimensions were confirmed: acute care, chronic conditions, continuity of care and referrals, well care and health maintenance, psychosocial care, and patient records. CONCLUSIONS Physician performance is multidimensional, including types of physician-patient encounters and variation across dimensions, as demonstrated by individual practice. A conceptual framework for multidimensional performance may inform the design of meaningful evaluation and educational recommendations to meet the individual performance of practicing physicians.