BACKGROUND: Poorly performing doctors are a source of harm but do not commonly feature in discussions of patient safety. Few countries have national mechanisms to deal with these doctors; most opt for suspension and/or exclusion from clinical practice. This study reports on the 11-year experience of dealing with concerns about doctors' performance in the UK National Health Service (NHS). The aim of this study was to describe the frequency with which doctors were referred due to performance-related concerns, examine demographic and specialty differences, and identify the nature of the concerns prompting referral. METHODS: This observational study uses data collected by the National Clinical Assessment Service for each referral (n=6179 doctors) over an 11-year period (April 2001-March 2012) in England to examine the rate at which concerns about doctors' performance occur, understand differences in rates between practitioner groups, and changes over time. FINDINGS: The annual referral rate was five per 1000 doctors (95% CI 4.6 to 5.4). Doctors whose first medical qualification was gained outside the UK were more than twice as likely to be referred as UK-qualified doctors; male doctors were more than twice as likely to be referred as women doctors; and doctors in the late stages of their career were nearly six times as likely to be referred as early career doctors. DISCUSSION: The UK holds a consistently collected national dataset on performance concerns about doctors. This allows risk groups to be identified so that preventive action and early intervention can be targeted most effectively to reduce harm to patients. A feature of past handling of poor clinical performance has been late presentation and a lack of thematic study of causation.