The purpose of this study is ( 1) to evaluate whether an educational program for preclinical students has an effect on the knowledge of professional behaviour for clerks and ( 2) to study how preclinical students compare with advanced clinical students concerning this knowledge. The authors used an observer-rated test consisting of vignettes describing a dilemma concerning professional behaviour. Students' description of proposed behaviour was scored in one of three categories: unacceptable, acceptable or excellent. Preclinical students were compared with clinical students and the effect of an educational session on student scores on this test was assessed. The educational programme had a positive effect on student scores. However, the knowledge of professional behaviour did not generalize to situations that had not been the explicit topic of the session. A difference was found between preclinical and clinical students in two of the four vignettes: in one vignette clinical students had better scores, in another vignette they had worse scores. It is concluded that the educational programme had a positive effect on student knowledge of professional behaviour but there was no generalization. For more generic rules to sink in, probably many cases need to be discussed, with explicit attention paid to the rationales of these rules. Students who were advanced in their clerkships held different views on professional behaviour compared with preclinical students, possibly due to a socialization process.