Introduction: Many authors are concerned by students' moral reasoning not developing normally during medical education. Aim: This study is concerned with how the components of student' moral reasoning are affected by their medical studies. Methods: Ninety-two medical students were tested on entry into first year and on finishing third year, to determine evolutionary changes in their moral reasoning. Changes in their use of arguments specific to each stage of moral development were measured. Results: Significant changes were observed in the weighted global score (-18.14 ± 59.17, P = 2.8%). Changes in global score correlated with changes in stages of moral reasoning. The multivariate structure of moral reasoning was reorganised into two principal components, which, respectively, explained almost 82% (first year) and 72% (third year) of the total variability in scores. Moral reasoning stages characterized by law-and-order and social-contract/legalistic orientations proved important for explaining the variability in students' moral reasoning at the start of medical training, while instrumental-relativist and interpersonal-concordance orientations explained variability post third year. Conclusions: Students restructure their handling of ethical questions by using arguments with more instrumental-relativist and interpersonal-concordance orientations, rather than those of the more desirable law-and-order or social-contract/legalistic type. To assess better the skills required for moral reasoning, a more sophisticated approach is needed than that of a simple measure of improvement/stagnation/deterioration.