Resident doctors contribute significantly to the quality of undergraduate medical training and it is assumed that by participating in the process, they also improve their own professional competency. We decided to investigate whether there is evidence to support this assumption. Our hypothesis, the physician-as-teacher rule, stated that "A skilled teacher has an increased likelihood of becoming a competent clinician, than a skilled clinician has of becoming a competent teacher". We conducted a literature review to search for evidence to confirm or refute this assumption. Twenty-four articles written after 1990 were identified as relevant from 132 references we generated by searching Medline. The identified articles were qualitatively reviewed to identify key research conclusions and/or main discussion points. The findings from the review were collated and discussed. None of the studies showed specific evidence of how teaching results in improved professional competence. However, there was evidence that teaching ability correlated positively with the perception of clinical competency. There was also need for improved supervision and training programmes for residents in teaching skills. The review provided evidence that teaching influenced the perceived professional competency of physicians positively. Physicians who were perceived as competent were those who taught effectively, and who had a basic understanding of teaching and learning. The review shows that training in teaching is essential for physicians, and that further research is still needed to demonstrate the effect of good teaching on professional competency.