<p>Abstract</p> <p>Background</p> <p>Professionalism is a difficult construct to define in medical students but aspects of this concept may be important in predicting the risk of postgraduate misconduct. For this reason attempts are being made to evaluate medical students' professionalism. This study investigated the psychometric properties of Selected Response Questions (SRQs) relating to the theme of professional conduct and ethics comparing them with two sets of control items: those testing pure knowledge of <it>anatomy</it>, and; items evaluating the ability to integrate and apply knowledge ("<it>skills</it>"). The performance of students on the SRQs was also compared with two external measures estimating aspects of professionalism in students; peer ratings of professionalism and their Conscientiousness Index, an objective measure of behaviours at medical school.</p> <p>Methods</p> <p>Item Response Theory (IRT) was used to analyse both question and student performance for SRQs relating to knowledge of professionalism, pure anatomy and skills. The relative difficulties, discrimination and 'guessabilities' of each theme of question were compared with each other using Analysis of Variance (ANOVA). Student performance on each topic was compared with the measures of conscientiousness and professionalism using parametric and non-parametric tests as appropriate. A <it>post-hoc </it>analysis of power for the IRT modelling was conducted using a Monte Carlo simulation.</p> <p>Results</p> <p><it>Professionalism </it>items were less difficult compared to the <it>anatomy </it>and <it>skills </it>SRQs, poorer at discriminating between candidates and more erratically answered when compared to <it>anatomy </it>questions. Moreover <it>professionalism </it>item performance was uncorrelated with the standardised Conscientiousness Index scores (rho = 0.009, p = 0.90). In contrast there were modest but significant correlations between standardised Conscientiousness Index scores and performance at <it>anatomy </it>items (rho = 0.20, p = 0.006) though not <it>skills </it>(rho = .11, p = .1). Likewise, students with high peer ratings for professionalism had superior performance on anatomy SRQs but not <it>professionalism </it>themed questions. A trend of borderline significance (p = .07) was observed for performance on <it>skills </it>SRQs and professionalism nomination status.</p> <p>Conclusions</p> <p>SRQs related to professionalism are likely to have relatively poor psychometric properties and lack associations with other constructs associated with undergraduate professional behaviour. The findings suggest that such questions should not be included in undergraduate examinations and may raise issues with the introduction of Situational Judgement Tests into Foundation Years selection.</p>