Does collaborative learning in asynchronous discussion groups result in enhancing academic discourse and knowledge construction? This general research question has been researched in a study involving 300 students, working during six months in 38 electronic discussion groups. The transcripts of the discussions were coded and analysed to test hypotheses related to the impact on knowledge construction. Coding of the units of analysis was based on the models of Fahy, P. J., Crawford, G., Ally, M., Cookson, P., Keller, V., & Prosser, F. (2000). The development and testing of a tool for analysis of computer mediated conferencing transcripts. Alberta Journal of Educational Research, 46(1), 85-88 and Veerman, A., & Veldhuis-Diermanse E. (2001). Collaborative learning through computer-mediated communication in academic education. In P. Dillenbourg, A. Eurelings, & K. Hakkarainen (Eds.), European Perspectives on Computer-Supported Collaborative Learning. Proceedings of the First European Conference on CSCL. Maastricht: McLuhan Institute, University of Maastricht. The results confirm that students in the discussion groups are very task-oriented and that higher proportions of high phases of knowledge construction are observed. Significant increases in the cognitive interaction, task-orientation and higher phases of knowledge construction are detected. Group size is a significant interaction variable. Discussion in smaller groups reflects larger proportions of higher levels of knowledge construction. The results point at the critical impact of structure in the task environment. In the discussion section, methodological issues are presented. The article concludes with directions for future research and some implications for instructional practice.