OBJECTIVE: To examine the performance assessments and cognitive function of practitioners referred to the National Clinical Assessment Service (NCAS). DESIGN: Retrospective observational study. SETTING: Practitioners referred to NCAS for performance assessment due to suspected performance problems. PARTICIPANTS: One hundred and nine practitioners over the age of 45 years referred to NCAS between 1 September 2008 and 30 June 2012. MAIN OUTCOME MEASURES: Reasons for referral of practitioners and their characteristics; details of their assessments including screening for cognition using Addenbrooke's Cognitive Examination Revised (ACE-R); outcome of the process. RESULTS: Reasons for referral included 'clinical difficulties' and 'governance or safety issues'. Eighty-seven practitioners scored above 88 on ACE-R. Twenty-two were found to have an ACE-R score of ≤88. On further assessment, 14 of these 22 practitioners were found to have cognitive impairment. The majority of all practitioners were found to be performing below the expected level of practice for someone at their grade and specialty. Of those scoring ≤88 on the screening, only seven continued in clinical practice. CONCLUSIONS: A high proportion of practitioners scoring poorly on ACE-R were found to have cognitive impairment following detailed neuropsychological testing, the youngest aged 46 years. Many were working in isolation. Nearly all practitioners scoring poorly on ACE-R were international medical graduates; reasons for this are unclear. Performance assessment results showed persisting failings in the practitioners' record keeping and in their assessment of patients. Our findings highlight the need for increased vigilance and training of responsible officers to recognise performance problems and emphasise the importance of comprehensive assessment.