BACKGROUND: The predictive validity of postgraduate examinations, such as MRCGP and MRCP(UK) in the UK, is hard to assess, particularly for clinically relevant outcomes. The sanctions imposed on doctors by the UK's General Medical Council (GMC), including erasure from the Medical Register, are indicators of serious problems with fitness to practise (FtP) that threaten patient safety or wellbeing. This data linkage study combined data on GMC sanctions with data on postgraduate examination performance. METHODS: Examination results were obtained for UK registered doctors taking the MRCGP Applied Knowledge Test (AKT; n = 27,561) or Clinical Skills Assessment (CSA; n = 17,365) at first attempt between 2010 and 2016 or taking MRCP(UK) Part 1 (MCQ; n = 37,358), Part 2 (MCQ; n = 28,285) or Practical Assessment of Clinical Examination Skills (PACES; n = 27,040) at first attempt between 2001 and 2016. Exam data were linked with GMC actions on a doctor's registration from September 2008 to January 2017, sanctions including Erasure, Suspension, Conditions on Practice, Undertakings or Warnings (ESCUW). Examination results were only considered at first attempts. Multiple logistic regression assessed the odds ratio for ESCUW in relation to examination results. Multiple imputation was used for structurally missing values. RESULTS: Doctors sanctioned by the GMC performed substantially less well on MRCGP and MRCP(UK), with a mean Cohen's d across the five exams of - 0.68. Doctors on the 2.5th percentile of exam performance were about 12 times more likely to have FtP problems than those on the 97.5th percentile. Knowledge assessments and clinical assessments were independent predictors of future sanctions, with clinical assessments predicting ESCUW significantly better. The log odds of an FtP sanction were linearly related to examination marks over the entire range of performance, additional performance increments lowering the risk of FtP sanctions at all performance levels. CONCLUSIONS: MRCGP and MRCP(UK) performance are valid predictors of professionally important outcomes that transcend simple knowledge or skills and the GMC puts under the headings of conduct and trust. Postgraduate examinations may predict FtP sanctions because the psychological processes involved in successfully studying, understanding and practising medicine at a high level share similar mechanisms to those underlying conduct and trust.