The use of online discussion is increasing in medical education curricula. Few studies have explored factors that affect patterns of participation in such an online learning environment. In this study, we examined how medical students interacted with one another in online discussions of end-of-life care during family medicine clerkships. Based on factors that affect the quality of online discussions as identified in the literature, we developed a coding scheme for comments that facilitated social interactions (social presence) and learning (cognitive presence). Our study was based on available transcripts from discussions including two faculty and 42 students. Participants created social interactions by greeting one another, referring to students' names, and connecting their postings to previous comments. While faculty prompted student discussions and posed questions, they rarely highlighted learning points, corrected student errors, or summarized discussions. Students offered multiple perspectives on end-of-life issues based on personal and clinical experiences. However, they tended to share existing online resources without much interpretation or explanation. Most comments by students lacked critical thinking skills in linking evidence from the literature with ongoing discussions. Students may need direct modeling by faculty to learn how to use existing resources to support statements, pose critical questions, and justify proposed solutions.