The objectives of this study are to explore the use of asynchronous discussion groups during medical students' clinical rotation in paediatrics. In particular, the impact of role assignment on the level of knowledge construction through social negotiation is studied. Case-based asynchronous discussion groups were introduced to enhance reflection and critical thinking on patient management and treatment, and to offer an exercise in evidence-based medical practice. Groups of approximately 4-5 students were asked to discuss 4 authentic cases during clinical rotation in paediatrics. 49 students interning at the paediatric ward participated in this study. With respect to role assignment, differences between groups (1) with a student or an instructor as moderator and (2) with or without a developer of alternatives for patient management were explored. A content analysis was performed to explore the different levels of social construction of knowledge. The results of multilevel logit analyses show a significant difference in knowledge construction through social negotiation between conditions with a student moderator and conditions where the instructor is moderating, but only when a developer of alternatives is involved. No significant difference was revealed between student-moderated and instructor-moderator groups without a developer of alternatives. It can be concluded that when both the moderator and developer role are assigned to students, their contributions are more likely to reflect a high level of knowledge construction.