Introduction: Solo and nonurban practice, nonmembership of a professional group, and aging are all associated with underperformance and may be indicators of professional isolation. Although it may lead to underperformance in physicians, there are no clear measures to assess professional isolation. Methods: By mailed questionnaire, experienced performance assessors were asked to list markers of professional isolation. Results: A set of criteria and descriptors for professional isolation was generated, and included personality; solo practice; poor colleague relationships; outlier practice; MOPS or CPD failure; specialist in only private practice; stress, no relief, complaints, job dissatisfaction; locum, itinerant or part-time practice; cultural barriers; male gender. Discussion:The identification of physicians at risk for professional isolation may enable the prevention of poor performance.