In this paper we focus on online forums dialogic structure, within the general framework of “collaborative knowledge construction” that forums, potentially, allow, giving participant the chance of expressing their own ideas, to think about other participants perspectives and, in so doing, to build new knowledge. In particular we focused on the role that agreement and disagreement play as characteristics form of forums dialogic structure. We suggest some categories of analysis for this dimension and describe the results derived form their application to a sample of 70 notes. The descriptive analysis of the sample of notes revealed that they consist, for the most part, in messages expressing a personal position, more frequently an agreement than a disagreement, in any case a motivated position. This assumption of position refers either to the lesson subject or to previous note (sometimes to both), but most frequently to the former. We suggest that our results contribute to the investigations of the ways in which collaborative elaboration of knowledge takes places within on line forums.