This paper illustrates collaborative knowledge building through problem-based learning (PBL) in online threaded discussions. PBL has been one of the most popular instructional approaches in health education. However, few qualitative studies described the essential aspects of the PBL learning processes, such as self-directed learning, and its impact on the building of professional knowledge. Therefore, this study illuminates students' ability to select, evaluate and make use of learning resources, a form of self-directed learning, embedded in a social context. This study aimed to 1) reveal different kinds of resources acquired and presented by students during discussions and to 2) describe patterns of knowledge building that emerged from integration of the above learning resources and consequently led to professional knowledge building. Using qualitative research methods, I analyzed 751 text messages from a distance learning class. The results revealed seven kinds of commonly used resources in the discussion. Results also show knowledge building occurred through the process of 1) enriching conceptual understanding, 2) connecting textbook information and personal experiences, and 3) comparing theories and practical approaches.